I would immediately put Cardinal in my arena team if I had her. You don't need to do the speedrace anymore with Cardinal. Just put your other 3 teammembers in damagegear and Cardinal in the block debuffs set (to avoid stun, freeze and things like that) with as much def and hp as you can get.

I gave that a try, but unfortunately she still died. My fire knight progress is not great, so good immunity gear is hard to get, and my attempt at putting resistance/def/HP on her did not work out well as the kind of teams I fight seem to have very high accuracy.

I don't even know if you really need "good" immunity gear. The point is that her revive boosts TM to 100%, so you actually want her to be as slow as possible. You could arguably even put her in a completely non-upgraded immunity set, and have her be level 1, and then gear the rest of your arena team as much in glass-cannon mode as you can.

Your goal is to make sure the rest of your team dies before she can act. The immunity gear keeps her alive long enough for her to get the revive off, and then the rest of your team wakes up and wipes them out.

The champs you listed won't really work for this, but if you have Kael or Athel, along with Spirithost and Warmaiden - that might just work. You'd want to use a bit of an unusual build on them, since normally you want speed. In this case, you'd want to gear Spirithost/Warmaiden with a decent amount of accuracy - probably around 200-250 - and then ATK/Crit Chance/Crit Damage, with Kael/Athel being pure ATK/Crit Chance/Crit Damage.

If your cardinal is lvl 1, she dies from the AoE attack the opponent brings out in his first turn. A dead Cardinal can't revive anybody. So of course you need very good immunity gear on her and she needs to be lvl 60.

Forget about Cardinal...level up Klodd Beastfeeder make him 6 star,max him out..he is an amazing carry..shields,speed buff,accuracy buff,heals.He is top tier.

What does your account roster look like?

Who is your Campaign Farmer?

What is your Clan Boss Team? 

What Clan Boss Mode do you hit? 

How much damage do you do? 

What is your Dragon Team?

What is your Arena Team? 

If you share with me all of the above things, I can get a better understanding of who you should 6 star.

If you don't share with me all of the above things, I will just tell you to 6 star someone I like.

Out of all the heroes you listed - The hero I like is Steelskull.

I use Steelskull in my Clan Boss team against certain Affinity Changes.

I use Steelskull in my Skinwalker Faction War team.

I use Steelskull in my Doom Tower team.

Previously, I wasn't using Steelskull in the Doom Tower.

As of yesterday Doom Tower Reset, Steelskull made a grand entrance.

So yeah

Apologies - wrong set. I meant Untouchable, the one you can make from the forge. Should be able to craft a set pretty easily, and shouldn't even need to bother leveling any of the items nor the Cardinal.

One buff stripper, Serris or someone else, is all it would take to deal with the immunity/untouchable set... or indeed just killing the Cardinal if you have her so low level. 

I mean, unless I'm misreading the item set, the Cardinal is unkillable for two turns? Yes, of course, buff strippers will mess it up, but aside from that?

I can't beat DT scarab due to lack of shields, so crafting even 3*-4* untouchable sets is not on the table at the moment. I do have enough for a full immunity set, though it's not great, but that leaves her with too low defense/HP to survive nukes from the kind of teams I face. Swapping it for an untouchable at some point would instead leave her vulnerable to disables, so it might fail for that reason and I'm back to having to build resist up anyway. Regardless, I don't think I really have the nuking power to really take advantage of this strategy. Even with Seeker and Madam Serris, I'm struggling to one-shot anyone, let alone an entire team.

Anyway, to answer the question about the rest of the roster:

Arena team: Seeker (lead, 217 speed), Madame Serris, Athel, Spirithost (in that turn order). Occasionally I replace Spirithost with High Khatun lead if I think I can win the speed race, but since that's just a dice toss I tend to avoid speed teams. My AoE options are limited, I'm even considering investing in a Kael I found in a mystery shard a couple of days ago. The purpose of Spirithost is just to make the enemy waste their debuffs in their first round, but replacing her with another nuker might give me enough damage to take out the other team outright in some cases.

CB team: Seeker, Apothecary, Jareg, Athel, Frozen Banshee. It's really just about keepin Frozen Banshee alive placing poison for as long as possible. Athel is just there because I got nothing better and she can do a bit of damage with her 3-hit giant slayer procs. I can 2-key brutal with a bit of luck (or if I do it manually), but often I need 3 if my team AI decides to make bad choices.

Dungeon team: Athel, Madame Serris, Apothecary are my core. Doompriest, Shaman, Frozen Banshee and Seeker I swap in occasionally. Mostly I keep Shaman in there for safety in case I lose a champion due to some unfortunate crit procs or something, and my reviver options are limited (Cardinal and Mistress of Hymns are the others). Frozen Banshee I use on dragon mostly, Doompriest on ice golemn and minotaur to deal with the constant debuffs.

Fire knight team: Under development. Athel, Apothecary and Coldheart are given. I originally leveled up Coffin Smasher to 50 for this as well, but I don't know if that's a good long-term solution. I'm thinking just adding Seeker to let the other guys hit more (and he got a 2-hit A1 himself), and some form of support as the last one, be it Shaman or Doompriest.

Immunity is just a 2-turn block debuffs - it can be stripped and even if it isn't stripped, it definitely doesn't make you unkillable. I haven't tried Untouchable but see no reason why it would be any different in that regard, it merely adds the 40 resistance. 

Hm, somehow I got the idea that one applied block debuffs and the other applied block damage, but apparently not. Oh well, same issue then. Too much damage and all the resistance and debuff blocks in the world won't help.

Ah yeah. That is really badly worded, wow. Okay, ignore me then.