Plains Elementals

Ap00calypse

...are not attacking. Or rather, are attacking once in a blue moon. I've yet to find what triggers the attacks, it was Gideon's Vigilance once, other times they're just being skipped over during the attack phase. On the other hand, once Gruesome Slaughter is cast, it keeps being respelled as long as the Elementals are fielded = nothing but Chandra can touch you and you're royally screwed if the RNG doesn't pull any creatures, since the Elementals are frozen and she just nukes you, repeatedly.

Ap00calypse

Upd:

It's Gruesome Slaughter. Once cast, it becomes 'attached' to the tokens. They do not attack anymore, but just nuke unfriendly creatures during the attack phase. So, essentially, it is safe to exile any additional Gruesome Slaughter cards, because as long as the tokens are alive, the spell is active (glitched).