Phlafantasia, Dark of the day

This enters the battlefield with 20 +1/+1 counters, then put 18 -1/-1 counters on this. When this attacks, you may remove a +1/+1 and -1/-1 counter from this and if you do, put a -1/-1 counter target creature and put a +1/+1 counter on target creature. During your attack step, put a -1/-1 and +1/+1 counter on this. {5}, put a -1/-1 counter on target creature you control: Destroy target creature, then each other creature that player controls gets -1/-1. When this dies, transform this back.