Definitely  focus  on  CC.  As  someone  in  my  clan  said:  "If  you  can't  roll  with  ease  like  the  big  whales,  CC  your  way  to  the  top"

Bosses  being  the  exception  if  course.  

Yeah, that first wave, double Skartorsis and triple Serris, was really nasty, I had a lot of trouble with it as well... finally managed to get through it with Brogni in lead, so resistance aura, and enough resistance on most of my champions that most of the Serris debuffs failed. Even then it was tricky and took a while before I could kill an opponent - once one goes down and it's five on four, it gets easier. 

Of course, you don't have Brogni and not sure what you have available as high resistance champs. If your Trunda is built with high accuracy, probably your best bet is to go for stuns and restart as needed for good RNG on Forge Rhythm on that first wave, then Stag Knight, then you make sure you have enough burst damage (Lua? Zargala? Cupidus? DE?) to kill at least one opponent while they're stunned. But since those Serrises hit surprisingly hard, you'll still need heals/revives as well - possibly Scyl could do it instead of Arbi though, adding some extra stuns. 

The second and third wave are somewhat less bad, still tough but easier than the first so should be able to finish it then. 

Or an alternative idea, though it requires building a champ from zero: Basher. With his A2 that increases cooldown on all skills by two turns, he can buy you two turns in which the enemies won't do much useful as neither Skartorsis nor Serris has a very impressive A1, that should be more than enough to nuke a few of them down, then the rest should be smooth. 

thanks! i built him and passed it :)

I'll proceed to close the topic as it's been solved already :D