The Shalka Doctor

{x} {b/g} {t} Perfect Pitch - When being attacked, add one of the following counters to {x} attacking creature(s).
• Using a {b} mana will add a -0/-1 counter to target creature(s)
• Using a {g} mana will add a -1/-0 counter to target creature(s)
I think, therefore, I win.