It's easy to run out of Banish if you aren't use any skills or use skills that have a fast animation. Some skills, like Lucky Fist, have a cast time and slows you down. You may or may not make it out of the Banish.
I forgot if the bosses give verbal clues before casting it. I always kill them before they can do anything. x.x
If you use Shield Breaker on a gun weak monster, the damage dealt will still increase.