Slither.io Worm

Pay {g}, put a +1/+1 counter on Slither.io
Worm.
Pay {r}, flip a coin, if it lands on heads
Slither.io Worm gets first-strike until end of turn
Whenever a creature that was dealt damage
by Slither.io Worm dies, Slither.io Worm
gets X +1/+1 counters where X is that creatures power.