can you trade less defense , just for the crit rate ? If so , then yes ...then again it depends what defense is he at now and how would 50% put him at ? 

I like my helicath to be above 3.5k-4k def .. with 240++ speed

Assuming your CB team is relying on Helicath to absorb the stun (hopefully with block/remove debuffs from someone else), then you only need him to have ~2200 DEF (assuming you're running him as lead):

2200 + 400 (Heli DEF aura) = 2600 * 1.6 (DEF up from Heli passive) = 4200 = 80% damage reduction, effectively the soft-cap for damage reduction.

I'm sure you're well over that number, at which point the remaining question is how much of his shield do you need for him to survive. You should observe the runs you are doing currently, and determine where your run is ending. It should be at turn 50. If it is ending sooner, then he needs more DEF. If he survives to turn 50, then you drop DEF for CDMG and CRATE until he starts dying sooner, and establish the baseline you need to survive. 

Thanks for the answers, guys.

Unfortunately, right now, the only trade I can make is 50% defense for 100% crit rate (in his gauntlets). It will also increase his crit damage from 83% to 96%.

The CB team uses him and Demytha, so it has block damage all the time, and always runs until turn 50. At ultra nightmare Demytha is the stun target, and, curiously, at nighmare, Helicath is the target. So, none take damage, and both are properly cleansed afterwards.

He has 5,228 defense, without counting the aura, 6,796 with it. He would lose 721 defense, down to 4,507, 5859 with the aura. He has 249 speed, and would lose only one point, keeping the comp running intact.

Since he does not take any damage, I am strictly considering his damage output: would the 100% crit rate and the small increase in crit damage improve the overall damage agains the clan boss, despite the lost defense, in this case?

Looks like it would. Worth the switch to test for certain. :)

Zero question. Answer is an easy yes. 

Thanks for all the considerations.

I did two test runs against ultranightmare with the change applied. Helicath´s damage increased slightly, around 1 - 1.5 million more. Unfortunately, still not enough to two key it. But every bit of extra damage helps.

I wonder if Swai Firstborn has the potential to work better than Royal Guard. 

It'll likely be a wash. Suwai has better multipliers, but doesn't have the A2 that RG has. So she'll do more damage on her A1 and A3, but less on her A2 (despite the max-hp scaling reduction).

Yeah, I tought that would be the case. Guess I´ll have to wait for a better DPS champion to come along.

What's the rest of your team, out of curiosity? Even with just "okay" gear, you should easily be able to 2-key UNM. Are you not running a poisoner?

They are doing around 31-32 million per run. It´s Helicath, Demytha, Royal Guard, Occult Brawler and Battlesage. I think this last one is the problem, but I don´t believe that I have a better cleanser ou debuff blocker. Even tough her and Demytha together can keep attack up for the whole run after the first stun, which is nice.

I also tried with Rowan instead of Occult Brawler, and also as a substitute for Royal Guard, but the results are marginally worse.

A few thoughts:

1. Maybe post your roster? It's possible there are champs whose utility you are not aware of.
2. Have you tried just letting the stun land? It'll definitely involve some changes, but since Demytha is void, you can basically be assured she'll never be taking the stun. That means as long as you run a team lead who is "safe" to stun, you likely will end up with a significant improvement in damage. Since Helicath is green, you likely want someone else green to be said stun target.
3. Have you tried putting a Toxic set on someone? OB will likely get no more than 5 poisons stacked at any given time, and you'll get DEF down and Weaken from Heli and RG. That leaves three debuff slots that could be filled by someone in a Toxic set. You'll probably want to have it be on someone who has only one hit on their A1, since otherwise it'll take too many slots up.

Okay, here it is (the rest are only rares, and the vault only has dupes):

i

i

i

About the other suggestions:

2 - I find it really curious, because at UNM Demytha is the stun target, and at NM, Hellicath is the target. I don´t understand why, really. Anyways, I think it´s an option, if I can find a champion to replace Battlesage that would take the stun.

3 - Indeed. Maybe I could try toxic, maybe even with Battlesage to try to improve her damage output. I´ll look at my artifacts, see what can be done. 

Okay. So I think I have a perfect improvement for your team.

1. Drop Royal Guard, add Lonatharil. The Ally Attack should improve your damage considerably - despite the fact that you aren't really doing a "huge" amount of face damage, the extra WM/GS procs alone should be enough. If possible, put him in a Reflex set and turn off his A2. That'll allow him to essentially have a 3-turn cooldown on his A3. If that's not possible, then keep his A2 on - it's marginally better for damage than his A1. If books aren't really something you can afford to spend on him, then go with Fenax instead, and turn off his A2. Fenax is one of the hardest-hitting champs in the game, though he does much better on a team with CA or AA.
2. This one might be a bit trickier, but - if you can arrange for Demytha to be the person placing Block Damage on turn 1 of each rotation, and Helicath to be the one on turn 2, then you can use Heiress as your stun remover instead of Battlesage. You'll need her to be faster than whoever is being stunned, and you'll need to arrange to have her A2 be used on turn 1 of each rotation to remove the stun. You'll also need to arrange to have Demytha's A2 be used on turn 3 of each rotation to remove the turn 2 debuff. This has two big advantages - firstly, Heiress is a harder-hitting champ than Battlesage, and gets a free CA on both turn 1 and turn 2. Secondly, the speed up buff, assuming you can build it into your rotation, will allow you to gear your champs much more easily.
3. If you're not comfortable with the above strategy for your stun removal, you could actually just combine 1 and 2 - since Fenax is green, he can very easily just be your stun target. That lets you keep Royal Guard, and simply drop Battlesage for Fenax. Better yet, if you can drop Royal Guard for Lonatharil, you get the AA to further boost his output.

In fact, upon re-reading all of this, I think I would just go with option 3. I'm keeping the rest of the writeup there if you want to follow my thought process, but ultimately I think dropping Royal Guard for Lonatharil and dropping Battlesage for Fenax will give you a huge damage boost. You'll need to run Fenax as your lead champ, and on red specifically, the stun target will likely end up being either OB or Lonatharil, but that's alright. Until you have Lonatharil booked, you can probably just leave Royal Guard on the team. 

I´ll see to rank up and ascend Lonatharil and Fenax during the current tournament. I believe I can adequately suit Lonatharil with a reflex set. Would a retaliation set be ideal for Fenax?

Right now I have seven leg books, and soon I´ll get at least two more from tournaments and advanced quests. I am probably going to save them for the next clan vs clan, but, by then, I believe it will be possible to fully book Lonatharil. Unfortunately, however, I am completely out of epic books, so, it will take some time to be able to book Fenax.

I´ve tried some lineups without Battlesage, but still couldn´t find a way to make the stun land not at Demytha. I´ve also tried a toxic set with Battlesage. It increased her damage a little, but still not enough to reach two keys. It was only one try, though, so I feel some more tries are needed for a proper judgement. Anyways, I should try the set with other champions, to see how they fare.

Thanks for your suggestions. I´ll put them to use, and report when I try them.

After doing some spider runs, I managed to increase Helicath´s damage by 40% with an amulet. New faction guardians also increased Royal Guard and Battlesage´s attack by 10%. That did the trick, and now I can two key the monster barely, but consistently.

Yet, I am still planning to insert Lonatharil in the lineup, as well as try Fenax. And I am also thinking about giving Urogrim a chance in place of Battlesage.

Thanks for all the help.