sniper only helps with debuffs that don't have 100% chance of being placed so it won't affect deacon's a2. with all debuffs and similar effects, there's always a 3% chance of it being resisted regardless of how big the difference between your champions accuracy and the targets resistance is. the reverse is also true; no matter how low your champions accuracy, there's aways a 3% chance of landing a debuff. that's likely where your issue lies in regard to minotaur. 

your deacon does need some work done though. crit rate and damage are both utterly useless on him. the only stats that are important for him are speed and accuracy. rebuild his with the focus on those two. hp and defense are useful as well, albeit much less so, so try and build those up anytime you can't increase his speed. the other four stats you can just ignore completely.

beyond that, the advice i can offer you is limited. without knowing who you have available, it's hard to build a better arena team. post a picture of your full roster.

Another question I have in regards to building my arena team: how do I speed tune my team while taking in account of tm boosts... My inc atk buffer (SachI) has a 20% tm boost with her inc atk. She's running 233. So, should my debuffer (Deacon/Madame Serris) come in at 187+ speed in order to not get cut in line? I don't think that's right so I would appreciate any advice on this.

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Free gear remova started today so it's the perfect time to build my arena team 😉

My best advice for you is - stop trying. You won't ever win any fight against a "real" team with a 266-speed Apoth as your fastest champ. Conversely, you'll beat the easy 1-man defense teams without any extra effort.

If you truly want to be trying to win those speed races, you need speeds of upwards of 330 to even have a glimmer of a chance.

Stop trying??? Lol, not really an option. I'm pushing arbiter missions and I'm on the level 7 affinity bonus GH mission. And to finish I'm gonna have to get to level 10, so farming gold medals is mandatory. I've been playing  Raid long enough to know there's more than one way to skin a cat. I'm farming dragons 21 non stop looking for gear to speed my champs up. I completely took Apo out my arena team. Out speeding a speed team is not the only way to beat thoss faster teams... i know because I beat plenty of them. I'm just looking for improvements and GOOD advice. And telling me to quit is not good advice lol smh. There are improvements I can make that someone else might think of that haven't crossed my mind... a go second team, for instance. But thanks anyways for trying.

So I rebuilt my team, taking your suggestions into account. I got Sachi running 245 speed with no crit. Deacon comes in at 238 with 259 acc and no crit. Umbral is going 222 with 240 acc and no crit. And Kael is 193 with 100% crit and 212 crit dmg. Inc Atk/20% tm into 15% tm Inc/dec and dec def, into a two turn provoke lockout, and Kael smacking then cleaning up.

Definitely working better in the arena. Thanks for the advice. But I'm still curious if you can see any champs that I could sub in, or even a whole new arena team you could suggest from my roster, that might work better for me... 

i realise that this isn't the advice you want to hear, but krama is right. you don't have the champions nor the equipment to compete with most well-established teams and you're better off investing your time and resources into other areas rather than trying to make some infinitesimal improvements to your arena team. specifically in your case, i'd suggest focusing on levelling up maneater and painkeeper to make a budget unkillable clan boss team (linked below). i can pretty much guarantee that it will perform better than whatever setup you're using and i can absolutely guarantee that being able to clear unm and nm clan boss will be much better for your account than making your arena team slightly more effective. having said that, i'll still throw you what advice i can.

• make sure to double-check champions' skills before modifying their builds. unlike deacon, sachi does need crit rate because her tm boost requires a critical hit. because of that, she's not really a great choice here because of that. seeker would  do a better job performing the same role.
• with the setup you're running, you'll want umbral to act last in the turn order. the resurgent mastery is a popular choice in arena so you run the risk of having your cc debuff removed almost as soon as you place it if your nuker acts after her.
• speaking of mukers, rathalos should absolutely be able to outperform kael.

those tips should go aways toward strengthening your arena team, though i will again stress that that isn't really where you should be trying to improve right now. as one final piece of advice, stop farming dragon 21.  the slight increase in drop rates is offset by the increase in energy required. once you can farm 24, it's worth switching but until then stick to 20.

https://deadwoodjedi.com/speed-tunes/budget-maneater-unkillable/

Speaking from experience, I focused on Arena before Clan Boss, and the peeps telling you that focusing on CB first will advance your account substanially faster are 100% correct.  However, I thoroughly enjoy arena, and actively play all three modes.  I especially like 3v3 (Rare indeed) because it is a 3-part puzzle.  Maxing GH Bonuses early will help you with builds across the entire game.     

CP is right that Rathalos should outperform Kael as your nuker, but they are both attack based and therefore squishy.  Valla is one of the best epic defense based nukers in the game.  She self-buffs a Shield and an Inc Def buff; She removes shields from the whole enemy team, and her A2 (Crush the Wall) hits about the same as Skullcrown's A2.  Since she is defense-based, she is much easier to keep alive, and as an Epic, you will have books for her a lot faster than you will have them for Rathalos.

If you really want to try for a go-second team, especially as a 3v3 option, you could put one together with these toons, in this turn order: 

(1) Seeker for TM Boost & Team Inc Def (Passive):  A2 - A1  

(2) Zelotah as aura lead (highest Epic Res Aura for Arena): A2

(3) Madame for buff strip & debuff placement:  A3

• Note: If you are seeing a lot of Polymoprh, build her as a Hybrid with good Acc & Res.  One of my Madames is in Untocuhable.        

(4) Valla as Nuker & (back-up) Shield Removal:  A2.  

• Note: She hits harder with the Inc Def buff up, but depending on the setup, you may not have time to wait for the the A3 - A2 combo.

I ran a similar team in 3v3 for a very long time.  It was one of my original teams that got me into G1 3v3 back in the day.  Thenasil (Inc Def & Heal), Zelotah (Aura, Shields & Heal), Monster (Control, Shields, Secondary Damage) Valla (Damage). 

Good Luck & Have Fun!