Onapha, the Posteminent

Planesought (If this permanent is a creature, it can be attacked as if though were a planeswalker and damage is dealt to it in the form of -1/-1 counters.)
Whenever Onapha becomes untapped or is the target of a spell or ability, choose one —
• Creatures able to attack it this turn must do so.
• Target creature gets -1/-0 until end of turn.
• Target creature gains or loses either first strike or last strike until end of turn.