First of all, you need to find a room that contains a bunch of monsters and moving blocks. Like this one:


After entering, just leave the room through the door where you came in and make sure you do this, before the room is cleared. You will get XP, because some of the monsters will get crushed by the moving blocks and you can go back into that room and repeat this method over and over again, until you are satisfied or bored enough.


Later in the game are even better rooms which can give you insane amounts of XP in a very short timespan.


Best thing of all? You don't even have to waste ammo or swing your sword for it.

Have fun!