Savage Wiitigo

First Strike, Wither (This creature deals damage to creatures in the form of -1/-1 counters.)
Whenever Savage Wiitigo deals combat damage, put a +1/+1 counter on it.
At the beginning of your upkeep, if Savage Wiitigo hasn't blocked or been blocked by a creature since your last upkeep, put a -1/-1 counter on it.