Skullcrusher still needs to take the stun since he puts unkillable on himself.  The Tower can cover both of the CB AoE's with block damage, but the stun will one shot people in the later turns even with Brogni.

Its a very easy comp to build with rt sc doompriest plus 2 dps. Works on NM and unm, though affinities are a little tricky 

See if this site can help you out. There might just be the specific comp you have to use for uk.

https://www.deadwoodjedi.com/speed-tunes-9

You  probably  don't  want  to  use  a  Brogni.

That  will  make  him  unviable  for  other  content  due  to  low  speed  and  his  shield  mechanics  may  mess  it  up.

Which  composition  are  you  doing?

The  Slow  Skullcrusher,  Tower  is  definitely  the  easiest.  Deadwoodjedi.com has  a  link  to  a  reddit  post  that  is  very  helpful.

Do  you  have  a  Doompriest,  or  anyone  with  a  skill  that  places  Block  Debuffs  on  all  allies  on  a  3-turn  cooldown?

That  is  what  is  required.  Roshcard's  A3  needs  to  be  booked.  Skullcrusher's  A2  needs  to  be  booked  as  well,  and  if  you  are  using  a  Block  Debuffs  champion,  you  need  that  skill  to  be  booked.

Speeds-

Tower  245

Skullcrusher  174

dps  178  

dps  177

Then  if  you  are  using  a  Doompriest,  the  speed  is  186,  but  a  block  debuffs  champion  needs  to  be  173.  The  faster  dps  would  be  the  one  with  decrease  defense  or  weaken.

Skullcrusher  needs  the  lowest  HP  of  everyone.  Avoid  HP%  and  HP  stats.  He  also  needs  a  way  to  earn  back  HP.  That  means  youll  need  the  Lifedrinker  and  Warmaster  masteries  together.

It  should  work  fine  on  Void  NM  and  UNM.

For  Spirit,  Doompriest  is  required.  You  cannot  use  a  block  debuffs  champion.

It  should  work  fine  on  UNM  with  the  same  dps  as  Void.  I  do  sometimes  see  issues  with  Spirit  on  NM.  It  has  to  do  with  the  Decrease  Speed  debuff.  I  think  it  has  to  do  with  whether  the  clan  boss  lands  it  or  not.

For  Magic,  you  can't  use  Skullcrusher.  Youll  need  a  Gnarlhorn  instead.  The  speed  is  the  same  and  his  A3  needs  to  be  booked.  His  A2  also  needs  to  be  booked  because  sometimes  hell  prioiritize  that  over  his  A3.  Once  we  get  AI  preferences,  the  A2  could  just  be  turned  off  though.

It  should  work  fine  with  the  same  dps  as  Void  on  NM  and  UNM.

For  Force,  youll  need  a  block  debuffs  champion.  You  cant  use  Doompriest.  I  recommend  not  running  Magic  dps  because  it  becomes  more  difficult  to  work  with  stun  targets.  I  run  my  Tower  and  Grizzled  Jarl  (my  block  debuffs  champion)  in  Immortal  to  make  sure  they  have  more  HP  than  Skullcrusher.

Otherwise,  it  should  work  fine  on  Force  UNM.  It  doesnt  work  on  Force  NM.

As  I  said,  there  is  a  guide  link  to  Reddit where  a  guide  is  located.  Check  that  out  because  it  may  answer  some  more  of  your  questions.

https://plarium.com/forum/en/raid-shadow-legends/674_game-discussion/211882_anyone-use-cb-unkillable-comp-w--tower-and-skullcrusher/

Here  is  a  link  to  another  post  about  the  comp.  It  may  help  you.