Killing a monster is easy on lower levels, but if you find yourself wanting to kill monsters faster than it's time to start farming for kills.

Take yourself back some levels or however many levels back you feel comfortable with seeing the kill rate faster and faster, then farm for kills while in-game. Stay in the boss level (or any higher level area you are at), close your game session out completely (as the game's mechanics work even while you're not playing the game).

Doing this will allow your characters to defeat monsters in previous levels while you are still offline but in the boss level when you go to reopen the game. Staying in a higher level area in-game as you notice where it takes more time to kill a monster in-game will just soak up your time instead of optimizing it.