I  think  a  good  team  would  be-

Ninja

Sepulcher  Sentinel

Frozen  Banshee

Gravechill  Killer

Jareg

You  will  need  to  rebuild  your  champions  though-  your  Ninja  build  alone  is  completely  off.  Not  enough  Accuracy-  and  you  should  focus  on  that  and  Attack,  Defense,  and  Crit.  Rate,  along  with  more  Speed.

My j.a.e.r.g Passive didnt work 

i

Elenaril 

Here's the team I would go with:

Sepulcher Sentinel - *make sure* she has her A2 booked, and speed tune her to be on a 4:3 rotation so that she can use her A2 every cycle. In a perfect world she'd be tuned to be acting last and using her A2 on turn 2, but that's difficult to pull off, so realistically just make sure she uses her A2 on turn 3 to block the stun.

Sandlashed Survivor - because sepulcher can't be using her DEF up on turn 1, you need someone else for DEF up. Sandlashed does that job by extension - you'll want to have your cycle for Sepulcher such that she uses her A2 on her turn 4, and then Sandlashed will use her A2 on turn 1 to extend the DEF up. If everything goes as planned, that'll keep the DEF up on all three turns, though the tuning is important here. She also has the added benefit of her ally protect passive.

Frozen Banshee - the meat of your team. You'll want her on a 1:1 rotation, with her A3 being used on turn 1, and disabling her A2.

The two things that team is missing is counterattack and leech. I don't recognize any champs from your list with those, but if you have them, they'll fill out the rest of the team.

I see an enerail build, not a ninja build.

You are not ready for NM boss, you need to be focused on Brutal 12.3 (assume u r farming) and Dragon20 (you probably are not here yet).

You lack the 6star champs and artifacts needed for NM CB.

You have 5 6star champs: 4 attack heroes (bad balance) + valla (not really a great choice)

Based on this put FB on backburner, you have 2 posioners already and posion sensitivity is the most overrated debuff in the game.  Put Gravechill in vault, don't waste any more chickens there.

6star Apothecary, Sethalia & Melga.  These 3 will carry your dungeon progression.  Better gear is what you need right now.

Yeah-  definitely  didn't  see  Sandlashed-  she  is  a  much  better  option  than  Gravechill.  Was  at  a  loss  for  the  last  champion.

This Good WorkWorkThisWorkThis

i

WorkThisWorkThis

You need your clan boss champions to have masteries.  Frozen Banshee be above Kael/Ninja with the number of turns you are probably surviving if you have her in lifesteal with enough accuracy.  That's assuming you have her and Kael booked and you didn't waste books on Ninja.  If you did waste those books and he's booked out, you should use him.  Your team has no defense though.  You will not do damage to the clan boss if you cannot survive.  You definitely want Jareg in there.  Then get the champs you are going with to 60 and get masteries on them.  There is no reason to be running 3 damage dealers unless you are planning on always being killed early.

This.

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This work 

To.low.acc.and.def.apo.frozen.

i

Speed too

I like the recommendations from Destroyer for Frozen Banshee, Gravechill, Apothecary, Sepulcher and potentially Jareg or Ninja. That combines several debuffs that hit CB really effectively.

But you'll really want to try and regear the champs I'm seeing there.

For Nightmare, you're looking for 200 accuracy to get the debuffs to properly land. You want that aat least on Frozen, Gravechill and Ninja to get the Poisons and HP burns to stick. Then you want the proper speed, 171 or higher for all the champs. Then shoot for 3k defense or so on everyone.

It's nice you've got your Apothecary's crit rate to 100%, and to increase his effectiveness further, you want to try and get his crit damage up too. The more crit damage, the stronger his heals. Some of these might possibly seem difficult to hit with your current gear, but Perception gear can be a huge help to hit proper accuracy and speed numbers! The forge is amazing for the mid game!

You'll also want to get all these champs to 6 stars so you can benefit from the the Warmaster mastery and it'll really help your CB damage tremendously!

I  don't  actually  see  Snadlashed  in  the  roster-  so  I  am  a  bit  confused.

I  would  think  Jared,  FB,  SS,  and  Ninja  are  the  definite  options.  I  don't  know  about  the  fifth  choice-  since  Gravechill  doesn't  really  bring  that  much  to  the  table.

Quoting myself... lol

Lots of good advice for you from everyone, but you should ignore it all for at least a few weeks.  It's terrible for where you are at in the game.  What I said above, basically ignored your request for NM CB help and redirected you to Dragon 20 progress.  This was before you posted the gear of your champs, which as I expected is not good...

Apothecary, Melga, Sethalia >> dungeon progress >> better gear >> progress everywhere

Will you eventually 6* FB, yes.  Should she be in top 5 now, nope.

Apothecary.  Speed and sustain.  CR helps heal, CD does not.  Prioritize Speed then def/hp/resist, then any CR it is a bonus.  You won't be able to get the other stats high enough going for 100 CR.

Melga.  You are early game, you need a reviver.  I don't have melga, but I've heard good things.

Sethalia.  Just a top teir support/dungeon carry everywhere except FK.

Jareg.  Probably would be 4th on my list, though getting very magic heavy....

1  -  Jareg  or  Sepulcher

2  -  WarMadien 

3  -  Frozen  Banshee

4  -  Apothecary 

5  -  Hope

-  All  in  Lifesteal  Gear

-  All  with  200  ACC  except  Apothecary

-  All  with  3-4k  DEF

-  All  with  over  171  Speed

-  All  lvl  60

-  All  Fully  Acsended

-  All  Fully  Booked

-  All  with  War  Master  Tier  6  Masteries except  Apothecary 

(Apothecary should  have  Gaint  Slayer)