Cosmic Horror Gangi'el (POTD-EN029) [Power of the Duelist]

If you're Tributing a monster on your side of the field that is owned by your opponent, this card can be Normal Summoned with 1 Tribute. Once per turn, you can place 1 A-Counter on a monster on your opponent's side of the field. (If a monster with an A-Counter battles an "Alien" monster, it loses 300 ATK and DEF for each A-Counter during damage calculation only.)