Suddenly remembering an anime scene but don't know the anime name

If anybody know, I remember there was an old anime I watched on animax around early 2000, there's this mysterious guy with a black gun(not ordinary gun, either a long revolver or rifle) that always show up when MC had trouble defeating a monster, and the guy while preparing his gun he said a cool prayer while flicking a bullet with certain color on the back of the bullet before he load it and shoot (sometimes it's combination of number of different color bullets depends on the monster strength) and one shot the monster and dissapear.