I just figured I'd post that the Crusader's Crossbow contract is actually extremely easy to do on Mannpower. If you go to Hellfire (which has lava pits outside both spawns) and get the Regeneration powerup (the one that looks like a recycling symbol) you can jump into the lava and you will both take damage and regenerate health EXTREMELY quickly. The regeneration from the powerup isn't strong enough anymore to stay in the lava indefinitely, but you can stay in there for a good 20-30 seconds before you have to leave.

Alternatively, if you're playing against tryhards, you can use the grappling hook to do self-damage. The middle point on Hellfire is up high enough that you take roughly 80-90 damage if you fall from the rafters above it.

Strangely, the Brass Beast contract does NOT work the same way - you need to take actual player damage for that to trigger.