Definitely not Warmaster if your intention is using him in arena. WM is great for dungeon bosses and CB, but it's almost completely uesless in arena.

Whether you get Helmsmasher is entirely dependent on whether your team is running a reliable DEF down comp. If you can consistently get DEF down on your target before MK attacks, then you don't need Helmsmasher, because their DEF will already be crippled, and his A2 already ignores 50% DEF.

But most teams running MK are not expected to have him be acting before the enemy, which means you usually can't rely on them having reduced armor. I think there's certainly an argument to be made for running Flawless Execution, but my guess is that Helmsmasher will be more useful overall.

I am only using my Mountain King in 3v3 arena offense. Instead of Flawless Execution or Warmaster, I gave him Unshakable for more Resistance.

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I took helmsmasher since i use him in arena and want maximum face melting.

Thats a nice mountain king.

HelmSmasher,  Unshakable,  or  Elixir  of  Life  are  the  only  3  Tier  6  Masteries   I  have  seen  people use  on  a  PVP  M.K

-  HelmSmasher  is  used  for  more  general  damage  because it  gives  extra  25%  ignore  defense  when  it  procs.

-  Unshakable  is  used  for  High  Resistance  Set  Up's.

-  Elixir  of  Life  can  be  used  for  Shield  Set  Wearing  M.K's.

M.King  has  the  highest  Health  in  the  game  so  sometimes people put  him  in  a  shield  set  &  try  to  stack  up  crazy  high  amounts  of  Health  on  him.