You use a skill that Flinches, Tumbles, or Stuns. After you do that you will see a circle over a part of the boss. Tap it. It will only show the one closest to you. Tumble is necessary to break some parts. Knuckles and 1H Swords are better at breaking than everyone else.
ManaKhe- Lvl 285-300 Enchanter(gotta get dat Mblade and Stoneskin) ManaSel- Lvl 240ish Monk ManaLock- Lvl 147 Servant
As this is in 1h/2h section the best skill breaking pretty much all the time is spiral air. Sucky mp cost though at 300mp. Lv 10 hardhit works nicely. Spiral air and hardhit inflict 'flinch' (basically a short period to tap and break). Flinch is great, also stops skills cast when timed correctly and, unlike tumble, knockback, stun, doesnt make the boss go nuts and evade/block/dmg reduction 24/7.