I now consider this guide "complete". If you have questions unanswered here, post in the comments.

Intro:

Welcome to the full assailant guide. (Also referred to as 'mobs')

  This guide is here mainly because none of the Plarium games have a decent (or better) DB or theorycrafting websites, and it's here to try and cover up for that. You had vikingswiki.net (till they shut it down recently), you have the Vikings Wikia (which is horrendously lacking in real info, but together with VikingsWiki gave a semi-complete picture). Your best bet is WWM's YouTube videos.

In any case, These are all for Vikings and as such there are some slight differences.

Areas Covered in this Guide:

1. Why hit assailants anyway?
   
2. Which assailant do I want to hit?
3. So, what's going on behind the scene?
• Related stats and their effect on the report.
  
• How does assailant drop system work?
4. OK, so what do I need?
• Studies needed.
• Gear and Skills needed.
• Gems not needed (seriously, they do nothing).
  
5. How to properly hit the assailants.
6. Are you sure the gems do nothing?

1. Why hit assailants anyway?

     There are several reasons. Mainly for gear crafting (and then you want to hit the right one), specific gems,
     unlocking lvl 7 rss tiles (camps left after Alpha assailants) or event points.

2. Which assailant do I want to hit?

     This is somewhat over-simplified, but the following details the main goal of each assailant's gear set.

     Production Assailants:

• Lumber - Nomad. (Possessed)
• Stone - Renegade. (Bloody Duke)
  
• Iron - Vandal. (Bloody Duke)
  
• Food - Pillager. (Possessed) In this case Normal gear might be better.
  

     Troop Assailants:

• Spearmen (henceforth melee) - Fugitive. (Possessed)
• Cavalry - Heretic. (Black Inquisitor)
  
• Ranged - Saracen. (Bloody Duke)
• Knights - Executioner. (Bloody Duke)
• Siege - Rebel. (Possessed)
• Scouts - Disciple. (Barbarian Chieftain)
  

     Utility Assailants:

• Assailant hitting - Savage. (Black Inquisitor)
• Raids - Smuggler. (Barbarian Chieftain)
• Yielding/Gold Mining - Outcast. (Barbarian Chieftain)
• Hero Exp - Nord. (Barbarian Chieftain)
• Study - Virago. (Black Inquisitor)
• Construction - Paladin. (Black Inquisitor)
  

     Gems: (I mainly hit Alpha assailants. And those will usually make hitting normal ones unnecessary)

• Black Inquisitor - Abelsonite (+2/4/6/8/10% army offense), Sardonix (+3/5/7/9/12% melee def), Andesine (+3/5/7/9/12% knight def) & Painite (+3/5/7/9/12% scout hp).
  
• Barbarian Chieftain - Variscite (+2/4/6/8/10% construction), Cuprite (+3/5/7/9/12% ranged def), Hematite (+3/5/7/9/12% cavalry hp) & Hessonite (+3/5/7/9/12% scout def).
• Possessed - Turquoise (+2/4/6/8/10% study), Heliotrope (+3/5/7/9/12% melee hp), Rutile (+3/5/7/9/12% siege def) & Ammolite (+knight hp).
• Bloody Duke - Fluoite (+2/4/6/8/10% training), others.



The above gems are unique to Alphas and as such have "only" 5 ranks to them simply because you cannot get simple drops anymore once you have the stats needed to feasibly be farming Alpha assailants for drops. That said, technically anyone can hit them - no study needed.


• Normal gems (+1/2/4/6/8/10% offense to various troops, +1/2/3/4/5/7% march speed/capacity/yield speed, +1/2/4/6/8/10% army hp/def, +1/2/4/6/8/10% production of various resources) are spread over normal assailants.
  Distribution of which can be found on the Assailant Info sub-forum on the individual mob posts by Plarium.
  

3. So, what's going on behind the scene?

      Related stats and their effect on the report. (By order of importance, and rated from 1 to 10)

1. Hero Offense (or Hero's Fury) 10/10 - Increases the damage you deal and makes you obtain better rewards.
   The 10 is due to it's high numbers (it has a real effect), & accessibility.
   That said, like with {almost} everything in this game, don't expect the %'s to be accurate.
   
2. Critical Hit 10/10 - Gives % to deal double damage. This will almost always (with a small required minimum of hero offense) make you get a 1-step higher quality reward than an equivalent normal attack.
   The only reason is 2 and not 1 is said required minimum; Unlike Fury, the %'s here are not only accessible, but they also are "real" - within an acceptable statistic variance from their expectancy.
   
3. Enhanced Attack (henceforth "Sustained") 8/10 - Grants a stacking +15% damage for every time you've hit that specific mob within 1hour intervals in a row (can't hit another mob in between, not even with the hero's double). You get 5 possible stacks for free, and the rest 1 at a time for every skill point or study level done (up to 40 stacks for a reported maximum +600% damage).
   %'s are as accurate as usual but they do work eventually, and affect the rewards.
4. -mob defense 6/10 - Similar effect to increasing your attack (maybe in a multiplicative manner with Fury), and t certainly isn't reducing the defense - and not by the %'s mentioned.
   But hey, more damage and thus better possible rewards - so 6 despite the low and inaccessible %s.
   
5. -stamina depletion 3/10 - much like mob's defense reduction, the stamina depletion does not work the way they say it works. It reduces how much it costs you, sure - but the reduction that 75% reduction gives you isn't 75%, it's ~42.857%.
   So you will be paying 57.14% instead of 25%, or in human terms 228.5% of what you should have been paying if you actually had a 75% reduction.
   The accurate formula for the reduction of X% is TR(x%)=1-(1/(1+x%)). And since the % from all sources add up it just means that it is almost meaningless to 'hone' your reduction. Get what you must to unlock the real research and skills (all the ones that show higher on this list); In general it's better to hit on the days with the bonus (that adds +10% fake-reduction like all the other sources).
   For anything else that gives reduction %, don't rush it unless you feel you really really need it.
   But if (for example) you have 2 rare (I would even say uncommon) trinkets from Savages do NOT bother upgrading them to lgd - it will cost more than it will save you in the next 2 years of farming assailants.
   
6. +stamina regeneration 1/10 - if there was any ray of hope in the way depletion scales (it's not really depletion, it's actually "your stamina is worth x% more than the value you see on the bar" - and then you pay the full price), it was that it affected the stamina you get from the potions. If depletion had decent %, it might even be worth more than a stepping stone study/skill to unlock Fury and Sustained.
   So this ray is not there for regen. This is why only 1/10 - a pure stepping stone. This regen works only on the basic rate of 180stamina/hour and by the time you get meaningful % for it to allow you to have more sustained attacks (assuming you login once an hour to hit that assailant) it will only help vs lvl 1 assailants and you'll be way way way past those.
   
7. +'max' hero stamina 1/10 - The real "max stamina" is 50,000. You can go over your fake-max with potions without an issue, it just stops adding your stamina regen, which is already established as useless. Just like the above 5&6 - get only the required levels that you must, skip anything else. These studies will prove useful when you have no more study or construction left to do and you still want to score some points in Gaining Power tournaments.
   

      How does assailant drop system work?

• Rewards/item-drops have 2 categories - Miscellaneous (Time boosts, Gold, VIP pts, Tongs (aka "gem removers"), Inventory Expand, stamina potions, various production/attack/defense/hero attack usable boosts) and the items you actually want to farm (Assailant materials, gems and non-alpha mobs give also normal materials). You get an "actual item" on roughly 25% of the cases, ymmv.
• From the Misc category, VIP pts are "meh" till you hit VIP13 (useless after), the usable boosts will be largely useless and the amount of gold is too pitiful to speak of. If you get a decent size (500/200) stamina potion you can consider that as a "reduction of cost on hitting the assailants". Normal time boosts are better than gold, but nothing that will make you go "yay!".
  The citadel time boosts are nice, but really short duration to make you feel something remotely similar to "I'm fine with getting this instead of the gem/material I'm farming".
  Inventory Expansion might seem nice for the first 2-3 but afterwards it's just a waste. I doubt I will ever have enough items to fill a quarter of my hero's inventory.
  
• The quality of the item (in both categories) is determined by how much damage you cause, and the type of hit (normal/critical). For a Misc reward the quality might be expressed via quantity (gold/VIP pts, for instance).
• There is no hard-limitation on reward quality by the mob's level - but there is one since the mob can die before you stack enough sustained to deal the required damage. You can see lgd reward from lvl 5, btu you can't from lvl 1 simply because lvl 1 will die too soon.
  
• When a mob dies you get 1-3 "extra" items and a certain amount of rss from the same kind of the camp he leaves at his corpse. The quality of the extra will always be 2 levels lower than what they show you on the mob portrait (epic/rare for alpha, rare for 6, uncommon for 5).
• There are sweet-spots of sustained-% depending on mob levels to max your reward quality (higher isn't always better).
• Crits will almost always give you 1 quality above a normal hit does. The exception is before you have enough +dmg from Fury for it to take you from simple to common.
  

4. OK, so what do I need?

     Studies needed:

      In accord with how the stats work (see above) here is the study priority. No study is truly effective before 8/10:

1. Critical Hit 1 10/10. Unlike all the others, despite the convoluted (you'll see) study tree, this one doesn't force you into maximizing other bullshit-studies.
2. Hero's Fury 1& Critical Hit 2 9/10. This one will force you to get some trash studies but not a lot. Since depletion 1 10/10 requires depletion 2 9/10, that's the cut-off.
3. Finish Sustained 10/10 (you had it at 9/10 from step 2).
4. Fury 2 & Sustained 2 10/10. Plenty of trash studies to get here, but you have no choice. Leave Depletion 2 at 9/10.
5. Finish Fury 1 and Critical 2 10/10.
   
6. No other studies. I'm serious. By now you were forced to get way more than you want. Any study not already mentioned here, leave at the minimum possible level.
   

     Gear & Skills needed:

      Skills are pretty simple (above level 55 they do almost nothing):

1. Hero's Fury 15/15 - takes plenty of prerequisites but nonetheless this is #1 priority.
2. Sustained 15/15.
3. Finish Deterrence 10/10.
4. Finish Depletion from 10/15 to 15/15. If you want to laugh at how useless it is, look at the before and after promised % on the skill vs the actual cost of hitting lvl 5 or higher assailants.
   
5. Flip a coin between Regen and fake-max stamina.

      Gear (Savage, or Black Inquisitor) doesn't make a huge difference but if you have to choose:

       Weapon > Shield > Chest > Helmet > Trinkets.

     Gems not needed (seriously, they do nothing):

1. Nephrite - 5 Legendary gems of this will up your stamina/hour from the base 180/hour (more, since you have trash studies and skills you were forced to take) by the jaw dropping amount of 27!
   Totally worth +50% rss production (choose your type), or +50% army att/def/hp (if you use single-troop armies that's +60% def/hp).
   Okay, you got me, I was being sarcastic. These gems, even with 6 lgd ones won't make you sustain 1attack/hour on lvl 2 or higher mobs, and you can do that on lvl 1 mobs without them.
   
2. Malachite - Since all it does is let your regen work a bit longer, it's even more useless than the regen itself.
   

5. How to properly hit the assailants?

     For Alphas you sill get lgd rewards at max (600%) bonus so single hit till that and then finish off with one 15-20

     multi attack. For the others you just need to kill a few mobs of that lvl to find out when you get with a crit the

     reward quality you're after (i.e. you can go higher but for me 180% or 225% give lgd on lvl5) and kill at that %.

6. Are you sure the gems do nothing?

     Yes. With the tweaks to the Battling Assailants event(s) reward system you shouldn't get any checkpoints beyond

     the first one unless you already had other reasons compelling you to hit mobs already.

     In any case, just like before the changes, the gems:

• Won't make you gain more checkpoints in the events than without them.
• Won't allow you to farm mobs better without using potions.
• Will have no effect once you use potions.

     So it's like having 1 more cent on a 1mil $ balance. Is it more? Technically, yes; Practically, no.