This topic seems like it's just asking for someone to post a card that already exists
10
Shockblast. Mass Shockwave. Less damage efficient than RoF, but combined with a Permafrost Shield and a couple of Arctic Squids, you'll be able to insta kill more than 6 creatures in a game. Could lower cost and damage without reducing the OP combo
Something that doesn't have a large cost: 2
Superposition. Move all creatures to random places on their owner's field. Currently the only position dependent thing is Flooding. This kills a little over three quarters of all your opponents non water non immaterial creatures in conjunction with Flooding
I'm unsure if a Fractal for permanents would be innately OP. But Fractal Dim Shields, Fractal SoG, Fractal SoV, Fractal dial, and maybe Fractal pillar