Clamshell

This isn't a glitch, but it can help you out a little if you want to level up your familiars (and yourself, later on) without having to do the work yourself.

You either need a sword or weapon whose normal attack does damage in front and behind you (Shield Rod, etc) OR a shield that does damage (Medusa Shield, etc.).

Using a weapon only works if you have a turbo control. I don't know if those exist for 360 or not.

The shield trick is very easy to do. Just get a rubber band. You also need either a WIRED control, or be able to plug in your Xbox360 control into the system. The reason for this is because, even with a button HELD DOWN, if there are no NEW button presses, the control will still "time out" and "disconnect."

Go to the room with never-ending zombies near the very beginning of the game. Put the shield to the "B" button and rubber band it down so the shield is always out. Put your back to the "wall" and make sure the familiar you want to level up is out. Go do something. Come back. Your familiar (and you) will get 1xp per zombie. This is important later on when you're so high of a level that every enemy gives you 1xp each.

If you use a weapon, you can go out into the middle of the room, instead of against the wall. This may or may not be slightly quicker, since enemies are coming from both directions.