Oracle Text

Destroy target creature with converted mana cost 3 or less. It can't be regenerated.

Card Rulings

3/1/2010 A creature’s converted mana cost is determined solely by the mana symbols printed in its upper right corner (unless that creature is copying something else; see below). If its mana cost includes , X is considered to be 0. If it has no mana symbols in its upper right corner (because it’s an animated land, for example), its converted mana cost is 0. Ignore any alternative costs or additional costs (such as kicker) paid when the creature was cast.
3/1/2010 A token has a converted mana cost of 0, unless it is copying something else.
3/1/2010 If a creature is copying something else, its converted mana cost is the converted mana cost of whatever it’s copying.