3

Use Rollout on Clefairy. By the time Clefairy is KO'd, Rollout should have already gone for 3 or 4 turns, making it very powerful. When Miltank comes out, it'll be hit by Rollout, and (depending on your level) should do a good amount of damage against it, if not a one-hit KO. If it misses at any point, you get flinched by Miltank's Stomp attack, or your Pokémon is immobilized by love through Attract, though, Rollout will end, making this strategy dependent on luck.