I would say Ronda. The first 6* should be able to farm campaign (12-3 brutal) reasonably fast. Miscreated of course also benefits from being at 60, but much less. In general, HP and support champs are less urgent to upgrade, as they can do their job at lvl 50 for a while.

I'm going to disagree. Ronda needs books to be truly effective - unbooked her A3 is on a 5-turn cooldown, and her A1 and A2 are both single-target. As a campaign farmer she'll be quite slow. Ontop of that, you'll want to gear her in dungeon gear, which is typically not going to be lifesteal that she'd need to campaign farm.

My vote - pick between Athel or Elhain, and bring them to 60. My vote is Elhain - she has an AoE A2 and A3, which makes for pretty fast campaign clearing.

Also - I'll note that you're making the same mistake most newer players make. You're leveling too many people to 40 and 50. Stop doing that. Get either Athel or Elhain to 60, and once that's done, you can revisit who to do next. In the interim, don't just level random people.

Unbooked Ronda farmed campaign for me very quickly, actually, on that little referral account you saw me build for the UDK tests. I can't remember just how fast it was before I added any books, but it was pretty great. 

I would personally keep building that Athel though personally. Krama is correct in that she farms 12-3 Brutal slightly faster than Athel, however I find Athel the mucvh more useful starter, especially with Weaken for CB, and a 3 hitter for dealing with FK.

Anyways, get Ronda, Athel or Elhain to 60 (but only one of these)

How to Get Started in Raid: A little guide to help you get started and growing efficiently

Your Starter Champion to 60 is your absolute first priority
Using 1, 2 and 3 star champions farmable from campaign, 6 star your starter champion: they are your campaign farmer, and when they are60,they will be able to finish all of campaign, up to Brutal difficulty, for you. They will be farming “food” for you to level up and rank up your other champions. The best spot to farm food is Brutal  12-3, and as you level up your starter, they will be able to farm 12-3 with just the starter gear sets.

It is well worth  buying the masteries for your starter, too. This will make it easier for farming scrolls for your other champions too. This is where your first 800 gems should be spent, even before the gem mine. I often recommending buying masteries for your second main champ with gems as well.

Do not sell the free login Lifesteal set
Your starter champion needs it for the first few months: you can level up to 16 the glove, chest and boots. The remaining pieces you can leave them at 12. . An additional note, keep almost all 4 star and below pieces at no higher than level 12.

You NEED AoE Decrease Defense
If you don’t have yet a champion with the AOE 60% Decrease Defense debuff, Warmaiden is your friend. Having a champion that does AOE 60% Dec Def is huge. While many Epic champs are better, Warmaiden is a farmable rare from Campaign Stage 8, the Deadlands, that does that, and on a 3 turn cooldown. She can be booked for free, just using her duplicates as books until her A3 is fully booked.  She can be kept at level 50 and used across the game until you find one of the many excellent Epic AoE Dec Def champs that do more, and survive much better. If you have Tayrel, Seigehulk, Uugo, Dhukk, Stag Knight, Thylessia, don't build a Warmaiden.

Get a “Core Team” going
The Core Team is the engine that will power up your progress. Build a core team for general use, but primarily for clan boss use, looking for champions that:
1) 50% Decrease Attack
2) 60% Decrease Defense
3) 5% Poisons, HP Burn, or both
4) Various buffs that are helpful in clan boss and general progression: heals, increase defense, speed buff, counterattack, ally protection, etc. The Core Team will change over time, as you pull better options you may need to change some of the champions.

The power of 6 Stars
Focus on making a team of level 60 champs! Don’t make the mistake many beginners do of upgrading level 50’s aimlessly and endlessly. Instead, work on the food to make 60s for your core team, champs that will be long term use. 6 starring a champion unlocks the very important Warmaster and Giantslayer masteries, as well as the banner slot. This, combined with having your champion booked at least for their most important skills, will unlock their true potential.

Minotaur grinding for mastery scrolls
Fully master your core team: having most of your team at level 60 will make it easier to farm on Mino 15, the best place to farm for scrolls.

Enter The Dragon
Start Dragon progression. Your goal is to reach Dragon 20, where 6 star gear starts dropping. Lifesteal, Speed and Accuracy sets will help you build teams for all areas of the game, and also will help you gear up other teams for other dungeons. Leave those other dungeons for later though, as they all will require more specialized teams.
All of the above will help you progress in Clan Boss....

But why is clan boss important?
CB is a chance at having the best rewards in the game, on a daily basis, for free. Your ultimate goal should be to reach highest clan boss difficulty, UltraNightmare, within a year. Nightmare top chest by the time you've been playing 4-6 months. Each day it will give you a chance to get, daily, Sacred, Void and Ancient shards, as well as Legendary, Epic, Rare books, ascension pots, and gems. Plus you can get the exclusive gear sets: Immortal, and Cruel. Gear without energy to farm it is sneaky good for you.  

Bonus Videos:

Do's and Don'ts when starting Raid https://youtu.be/-Aiw3wk-3ik 

The Basics of Farming to Upgrade Champs https://youtu.be/Ho3yE7kSN6A  

Beating and farming Dungeon 20 with One Team To Beat Them All Wonderful video on how to build a good team that can farm anything. Answer things like What do I need to build an effective team? What skills and gear are important? What are the strategies and methods to beat each dungeon? https://youtu.be/ziwDFgAPoz8 

How to build a great speed/nuke team to climb to Silver and Gold Arena https://youtu.be/rfTNsJSPLOM 

I agree with harleQuinn and vote for Athel. Very usefull champ for the start, and later on, she may still be among the really used champs (no vault guardian) for faction wars and the DT hard secret room "magic affinity rares".

Thanks for the feedback folks, I actually upgraded my Ronda last night abit impatiently because i wanted to do it during the Champion Training Event for extra rewards but I don't think it was a bad choice though.

Yeah i read not long ago about getting my 1st 6* should be my first prio. Hence why I probably have a bigger pool of viable champs to choose from to upgrade and that makes it harder to make the better choice for a beginner 😀

I've already unlocked some masteries for Ronda but still a long way to go to fully unlock her so I take the advice and spend the gems needed to do that instead of trying to farm it. I've already unlocked all masteries for my Miscreated Monster though.

On another note, I have also fully unlocked my gem mine 😸

Currently I run Ronda(dmg), Frozen Banshee(poison), Gravechill Killer(poison), Miscreated Monster(support,dmg) and Bogwalker(decrease atk) as my CB team.

Planning on changing out Bogwalker(fully booked) for Coffin Smasher but will take awhile to get the rare books for him.

Yeah I had 4x 5* in my vault and got the 5th last night just for the record 😁