King Alastor is the final boss in Painkiller: Battle out of Hell, found in Shadowland level.
Even though you will get a lot of ammo in the beginning, Alastor cannot be hurt by normally being shot. However, there are glowing circles scattered across level. Stand in one of them and hurt yourself with Flamethrower (or any explosive that you have in arsenal). Giant golem will spawn somewhere near and you must distract Alastor for a while, so golem can reach him and stun him. When Alastor is stunned, take a chance to shoot him with anything that you have (even better if you have Tarot cards activated). If he is not killed, find another circle and repeat the strategy until he dies.