Tears of death one-shot used against multiple monsters

[Raymolacha]

In munchkin pathfinders with the "guns and razzes" expansion I have a scenario where I kicked open the door to Lamashtu. While I was calculating the combat scores, my opponent added a monster (deathweb 'undead') to the fight (using the special ability for Lamashtu enabling opponents to add 1 monster to the fight). I'm carrying the "tears of death" one shot that says "+3 to either side, no effect against undead".

I argue that I can still use the one shot since it's against multiple monsters and 1 of them isn't undead. Am I right?