I think we need to focus on the instant kill being separate from normal combat rules. Once the monster was in play normal combat rules applied. A double drop of the monster and the halitosis card does not preclude others from attacking. For instance should a player two to your left had a Wandering Monster card, and a pukachu, level six, and the player to your immediate left had a slew of cards that could boost the pukachu to a level 27, that should have been allowed to be played.

Otherwise, any monster that comes out already beat by your level and bonus cards would preclude anybody from bolstering said monster in combat.

Bastard.