If you are playing as a Slasher boss fights are extremely easy. You just have to dash to the boss when they try to attack/cast their skills as it knocks them up and stops their casting. Tried and tested on my Apocalypse.

Havent tried shooter but for a grappler it is manageable, you just have to time your interrupt skills before the boss cast his/her spells. If its on cooldown you can run far till its almost out. I awakened my grappler using that skill in which he grabs the enemy and to a throw ala Zangief.

It is really easy to time the interrupt skills/dash before the enemy turns red as they take about two normal attack rotation before casting skills, when they are red it takes about 3/4 of 1 attack combo till they cast so you have to be faster. The final boss on the second dungeon in awakening quest is a b***** as her attack hurts like hell especially if you get caught by that multiple hit spell.