<< ***warning possible spoiler**
This is by no means a walk through and I'm by no means an expert but I'll give you my 2 cents seeing I just finished the game. Outside the cathedral don't hang around the enclosed area where your ammo and armor pop up, in fact, get as far away as you can, it make take a few tries cause of the meteors, but I found it much easier to attack with rockets, cannon, minigun etc from a distance then move in to the enclosed area when you get a chance to stock up on goodies.
Inside the cathedral save save save, this was a bitch just direct as much fire as posssible, cannons first, at the boss and run like hell....good luck >>
FredLocks is right on. DON'T stay in the middle, go way off past the cathedral walls, then watch as the idiot bots go towards the middle and get blasted by the meteors (actually I found they only did slight damage). Then sneak up and as a hoard sees you and comes after you start dropping grenades on their arse. Every once in a while, when the creatures seem to diminish in number, take a run for the middle and restock on ammo and health, then get back to the outside.
Also, every once in awhile a creature pops up around your area, but not enough to cause much damage before you spot him and kill him.
As far as the cathedral, I will just say two things: find the pink secret and get the speed boots as often as possible. Their are a couple of other things that are unique to this boss, but I don't want to spoil everything.