Target creature gets -2/-2 until end of turn. It gets an additional -1/-1 until end of turn for each modified creature you controlled as you cast this spell. (Equipment, Auras you control, and counters are modifications.)
A creature that is equipped is considered modified no matter who controls the Equipment that's attached to it.
A creature with a counter on it is considered modified no matter what kind of counter it is or which player put it on that creature.
An Aura controlled by an opponent does not cause a creature you control to be modified.