The Xbox achievement guidelines for this are a bit misleading. You need to jump on a bullet five times in one go without hitting the ground - not just jump on five bullets in one level.
I found the easiest place to do this was on the altar level. Wait until the gun that looks like a large magnum appears and keep firing the fat bullets in straight succession. Once the ghosts start copying you, you should be able to easily jump on five bullets without touching the floor, though remember you can double jump too if you need to.