You can interfere with others’ combats in several ways, including:
- Use a one-shot card. You could help another player by using a one-shot to strengthen their side. Of course, you can “accidentally" strengthen the monster with it, instead.
- Backstab a player in combat, if you’re a Thief.
- You can also play a monster enhancer. These cards (usually) make a monster stronger . . . and give it more Treasure. You can play these either during your own combats or during someone else’s combat.
- Add a monster from your hand to join the combat, either with a Wandering Monster card or by using the special Undead rule.
- Curse them, if you have a Curse card.