A simple guide to grinding XP for your levels and weapons.
This is a pretty simple guide. It requires that you've completed the first zone of the game.
After you've beaten the first boss, simply go back through the door where you fought the first boss. You'll appear behind it, and since the boss can't turn around or go over steps... well...
And hold down the shoot key until:
There you go. Simple levels for you and your weapons.