Peter Jacob is the one who defeats it. Why is this any more rad than the achievements of The Chosen Many? Well, for starters, Jacob is a lad who's obviously in weak shape, and it shows: though he's faster, he runs out of breath even more quickly than his predecessor Roberto, an overweight man in his forties. Secondly, the boss is completely immuneto all the weapons Peter gets, and even mocks him for trying to shoot it. Thirdly, during the battle, the camera makes a mission out of showing just how shitless he is scared as the fight goes on. Even then, Peter's still more than the Guardian could chew, and he moves on to live sixty nine years in (almost) complete peace before he gets rid of the Essence he stole. It's also sweet revenge for this boss mercilessly killing another player character, Paul Luther the friar, in a previous chapter.
Having Alex channel the spirits of those involved in the tome smashing Pious's artifact during the final boss battle. With this task, they're finally free.
After surviving through most of the game with whatever weapons come to hand, finally getting your hands on the OICW assault rifle as Michael, able to dispatch even Horrors and Winged Guardians from a distance with ease due to its burst fire and automatic fire setting, not to mention a grenade launcher!