Flying, first strike, haste
Whenever Mirri the Cursed deals combat damage to a creature, put a +1/+1 counter on Mirri the Cursed.
If Mirri the Cursed is blocking or blocked by a creature that doesn’t have first strike or double strike, Mirri will deal combat damage to that creature, then get a +1/+1 counter, then (if the other creature survived) Mirri will be dealt combat damage.