When Vitals-Curser enters the battlefield, have a Shootout (Each player discards a card from their hand. The player who discarded the cad with the higher converted mana cost wins the shootout).
If you discarded a card with a converted mana cost of six or greater, target creature gets -3/-3. If you won the shootout, target creature gets -3/-3 until end of turn.
Chixelma as the plane name?
Wild west and death plane concept. Shootout is clearly evocative of what the card wants to do, but without guns I'm worried I may have to something that will just sound wacky, like spelloff.