Shrewd Hatchling [Mystery Booster]

Shrewd Hatchling enters the battlefield with four -1/-1 counters on it.
U/R: Target creature can't block Shrewd Hatchling this turn.
Whenever you cast a blue spell, remove a -1/-1 counter from Shrewd Hatchling.
Whenever you cast a red spell, remove a -1/-1 counter from Shrewd Hatchling.