Trust me when I say Angel Ray is worth waiting for. Big Bang isn't a very practical skill to use as you have to charge it (Press and hold key, then release). It's risky as it is because you obviously can't use potions while charging, and you have to storm into the center of enemies to unleash it at max potential. Angel Ray is definitely worth waiting for as it's quick, the strongest attack you'll ever use as a Bishop and mobs a fair bit.