Corrosive Preparation

Put a -1/-1 counter on target creature. If that creature would receive damage this turn, put that many -1/-1 counters on it instead.
If there are 3 or more creatures attacking you or a planeswalker you control, put a +1/+1 counter on a creature you control then Proliferate.
Flashback - {5}{g}. Reduce this cost by {1} for each creature attacking you or a planeswalker you control.