Who is your clan boss team right now, and can we see the rest of your roster too?

this is my current team and the rest of my roster 

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It looks like you have a pretty solid team already, though I am not sure if you have the right synergy.  I'm also keen to understand if any of your team is speed tuned., as well as how many turns on average your team lasts, plus who is the current primary sources of damage.  If I were to hazard a guess, I would say that your current strategy deals a lot of damage in a relatively short amount of rounds, with Ninja or Geo being the highest damage sources

Anyway, my thoughts:

• I'll start with the easiest one which is making sure that every champion in your CB team has Warmaster or Giant Slayer masteries, depending on how many times they hit on average.  If it is 3 times or more than you want Giant Slayer, otherwise you want Warmaster.  These masteries are a massive source of damage for CB and should be on everyone unless you have a very good reason not to.
• You have a couple of decent sources of decrease attack (Sepulcher Sentinel and Rector Drath), which is great.  Decrease attack needs to be on the boss 100% of the time, and if you find that it is not then you need to look at why.  It could be accuracy issues, or the champs cycling through their skills in an order that allows for overlap.  Just watch a full battle to confirm this, and also when you start to consider swapping champions out, you will need to find another source of this
• Looking at Ninja's stats, I think his defense is quite low.  With the 60% defense boost from Sepulcher, this only brings him to 2500, and ideally he needs to be at 2500 before the defense boost (which would put him at about 4000 with defense boost, which is maximum damage mitigation).  I would actually sacrifice some crit damage and attack to get another 800 odd defense added.  I also noticed that he has broken sets equipped.  This isn't normally a bad thing as stats should initially matter more than sets, but once you start to push up in CB you really need to start to levarage sets for a better overall result 
• I'd be keen to know the stats of the other champions in your team, because having a low defense will also result in the rounds ending quicker than they need to be.  The longer the fight goes on, the more damage the poisons do, as well as the more damage Geo does with his reflect.
• I spoke briefly above on speed tuning and I'd be keen to know the speed of all your current champions.  Sepulcher's increase defense is on a 4 turn cooldown, but is it up on all champions for both AOE attacks (i.e. is she running at 4:3 to CB).  Sometimes, just making sure that the champions are going at the right speed can dramatically improve the survivability of the run, and subsequently the damage output
• You have two sources of HP Burn, and one challenge with running Geo and Ninja is that the latters will override the formers.  This is a problem because then you cannot benefit from the 15% damage reduction from Geo, as well as the reflected damage source.  Again, this can be mitigated by having the right speed tune, with Ninja using his A2 right before Geo's uses his, so that Geo's burn is the last one on the boss prior to subsequent rounds, but even this can be skewered by Ninja's A3 which reduces the cooldown of the A2.  Ideally you want Geo's HP burn out for both AOE attacks.
• You don't have an ally protection champion in the mix yet, which is a key champion to extending the fight if you don't have an unkillable setup.  You have Toragi the Frog that does this, as well as brings poisons and the decrease attack buff too.  This could allow a revist of another champion.  The damage mitigation from Ally Protect is significant, and could mean another 10 rounds of damage output alone.
• If you found a way to bring other healing into the team, such as leech or the lifesteal set, you could remove Rector completely which would free up another champion slot.
• Kavalax is your source of poisons, and I won't pretend to know how good he is at doing this, as I don't have him.  Frozen Banshee can also do the job but also brings in poison sensitivity, that increases the damage of all poisons by 25%.  The question becomes how consistent is Kalvalax generating poisons on the boss over the course of your entire run, and compare that to Frozen Banshee.  You will want a consistent 7 odd 5% poisons up for as many turns as you can get (assuming 3 debuff slots already taken with poison sensitivity, HP Burn and decrease attack), and if both champions were equal in that count then I would probably use Frozen Banshee personally, simply because the final poison damage would be 25% more.  Legendary doesn't always mean better!

If he continues to use Kal, he will need another poisoner as Kal doesn't have the capability of keeping up more than 1 or 2 poisons at a time during any point after first couple of rounds. For Kal's ENEMY MAX HP ability - OP will want DEF DOWN and atleast 1 poison debuff to hit the cap which makes up about 50% of overall damage from Kal.

Maybe try to squeeze venomage inplace of rector, while finding some other source of healing or ideally a team heal on a 3 turn CD. VM will offer more damage reduction for all allies, places poisons,,ATK and DEF DOWN, while exploding poisons on the A1.

Thanks for all the help guys really appreciate it. My Rector's speeed is sitting at 229 speed, I have my Sepulcher Sentinel running at 225 speed to try and block all incoming stuns. I then have my Geo coming in at 192 just behind ninja so that it replaces ninjas HP burn so that Geos passive works. I may have to swap out Kalv because i do agree it is hard to maintain his poisons over the duration of the fight but definitely got excited to use him as my most recient legendary! thanks again for the help definitely will take all this info on board! Cheers! 

If Kalvalax is indeed only maintaining that amount of poisons then I'd straight up swap him with Frozen Banshee in a counter attack set.  In the current team Frozen will do less physical damage, but should do considerably more poison damage, which should translate to a damage boost with minimal changes. 

void:

Sepulcher sentinel, kalvalax, Venomage, Fahrakin, Deacon.

affinity: Sepulcher sentinel, Kalvalax,Venomage,Geomancer,Deacon

Venomage gives you daamage reduction and backup atk down. Sepulcher atk down and def up, deacon, leech and Fahrakin ally attack, hp burn and poison.

Geomancer really shines on affinity as everyone gets hit more.

Making a killable team without Toragi would be criminal. One of the very best if not best champs to have in these comps. 

I'd want to have SepSen/Toragi all the time here, for sure. SepSen will give us block debuffs permanently when speed tuned, attack down, the defensive aura and defense up, while Toragi will give us more attack down, some poisons, ally protect, shield and some heals. Just a ton of sustain there. 

Then Geomancer for sure. I like having Ninja freed up for most content, but he's also tremendous in CB, of course.

I'm fine with Kalvalax, though admittedly have no experience running him in Clan Boss, so I can't speak to how well he'd perform as a DPS vs. say, Ninja who we'd have to put in a little work in terms of skill ordering to avoid HP burn overwrite. 

The 5th is where I get a bit fuzzy, whether we should be jamming in another DPS or trying to find more sustain with something like Sandlashed (buff extension which would reduce speed requirements, and to a lesser extent her passive) or someone like Ghrush who will give us leech on an A1 and continuous heals. 

I don't know if Venomage fits here because I get the idea we're going to have the debuff bar jammed already, but more damage mitigation seems great. 

Would be curious to see what @harleQuinn has to say when she checks back in because Clan Boss comps are 100% her wheelhouse.

I think I'd go SepSen/Geo/Toragi/Kalvalax/Ghrush or another sustain champ. Deacon, I am usually leery of due to speed tuning/buffs but he could make a ton of sense due to leech, and again lowering speed reqs.

My two cents:

• Geomancer (hp burn, damage reflect, weaken) - team lead on green
• Ghrush (leech) - team lead on blue, turn off A2 and A3
• Fatman (ally attack, def down) - make sure to sync a2 such that you don't overwrite geo a3
• Giscard (atk up, def up, atk down)
• FB (poisons) - team lead on red

So, here's the thing with my comp - you have no stun block. That means you're relying on your team lead to be the person being stunned. That'll invariably eventually stop working properly, but the nice thing is, the only person on your team who actually needs to be tuned properly is Giscard's A3 being on turn 1 and him being slowest - so hopefully as long as he doesn't get stunned, you'll be okay.

Sepulcher Sentinel would keep us affinity friendly/worry free about the stun at a 4:3 on UNM. 

Yep - but no way to fit her into the team without losing one of the other important parts. I guess you could argue that Giscard could be dropped - but I'm leery about suggesting to OP to run without DEF up, as that's a pretty significant loss in damage to re-gear to support it. Not to mention Giscard does ATK up too.

(and yes, I know, SS does DEF up, but you need to use it to block the stun, so you don't get it for one of the attacks)

Though - good point on affinity. My comp obviously fails on green due to SPD down.

Pingping!

Okay, some folks want a lot of changes. And if they do, they need to be recommending Toragi, who is the best champ for a killable team in the entire game. If you want an example of what makes him so good, here's a wonderful video showing a team that features him, although this isn't the tune I am recommending: https://www.youtube.com/watch?v=gyh-Fy8VDQY Ally Protection is just broken, as you can see in this video, and we're going to make use of that.

Then we add in Sepulcher to keep ourselves totall affinity friendy and deal with stuns. She also keeps the Increase Defense going. However this is impossible without a buff extender!

Thankfully we also have Sandlashed Survivor to be extending our buffs! She is going to come in and just keep all kinds of buffs going.

We can pair these with you current DPS to create a fully affinity friendly, stun friendly, ally protect team that should last a long time, and work on UltraNightmare and Nighmare. Here is the speed tune for this:

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You can play with this team yourself over at the Deadwood Jedi calculator, the speeds can be tweaked here and there minutely and still have the correct buffs up at the right time. https://deadwoodjedi.info/cb

And here's a BETTER option, to get your Ally Attack AND Defense Down, as well as one of your DPS champs going 4:3 for extra damage, and buffing the rest of your team.

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As you can see, this tune will do WORK for you on CB. If you can build these champs with Solid HP/Defensive numbers while hitting these fairly attainable skills, you'll be top chesting UNM just about as soon as you get it going. (Maybe a 3 key, maybe 2....depends on your gearing ofc)

Good luck, and let me know if you have any questions making this tune work! 

(If you're having issues viewing the tunes and such, just save the images They're high rez enough, the forums just have display limitations -_-

Well this ended up being considerably more difficult/less simple than I thought it'd be. Looks quite unpleasant to tune. 😇

This is a really, really nice way of sequencing Fahrakin and Geo, too. Get full value and barely a negative interaction with Geo's passive. Awesome stuff.

With him going 4:3 you do a lot of things: 1) Increase Dec Def uptime 2) Prevent overriding Geodude's burn/passive 3) Keep crit rate/crit damage buffed 4) Ally Attack more 5) More poisons