The 1st level is probably the easiest to do this. However if you are struggling do it on New Game+ makes it a lot easier. You can make your character start at level 40, unlimited abilities and you can change your own damage modifier allowing you to kill everything in almost 1 hit. The only tricky bit is the end of the level, but i used the journalist G ability (ghost one) to just run past everything and back up the stairs and then turned around and killed them all. You could do the same with the Priests invincibility skill probably.