MonkCombat.cs line 77->
if ((Player.CurrentHealthPct <= 0.50 || (Player.IsIncapacitated && Player.CurrentHealthPct <= 0.90)) && CanCast(SNOPower.Monk_Serenity))
first is the trigger at which hp % the bot should use serenity, 2nd is same as first but has higher hp% if player is stunned/frozen etc and third just checks if you can currently cast it.
If you are looking to cast it when ever possible just change the line to:
if (CanCast(SNOPower.Monk_Serenity))