would it be possible to take him out using hell wave or the such? or atleast damaging him enough that you could battle him?
If you've got time, you could do Game Hunter's strategy of running round the outside of the Cathedral, throwing combat knives at him, then picking them up again, and repeating.
However, it'll take a lot longer if you aren't on Angel of Max Carnage, and it's not a very efficient strategy and is only really an option if you want Longinus Diamond (which involves clearing Unholy Cathedral on UV or N! without Brute, and therefore Berserker).
There's also the Hatred Skull, but you'll still need a chainsaw, several large medpacks and red armours to have a chance at killing him before berserk runs out.