If you're going to farm it using Sorcerer, then Holy Focused Bolt all the way. Even with that low Magic, you'll be doing more damage than your more-frequently-than-not interrupted big spells. Make sure to use 4 Demon's Periapts, and go solo, with Pilgrim's Charm.
I personally don't think periapts or any of that stuff is necessary. I can do sub-8min Urs without even using Assassin augments. In fact, I'm doing a pure mage right now at 96 using Dragon's Risen and I've been too lazy to get other class augments so far and the highest level pawn I'm using is my own and she doesn't have good augments either.
If you just go at night with Bloodlust, that should already be more than enough with 300 base MAG. You'll be coming in about as strong as I was when I started and have more defenses to boot (not that it should matter, since Ur shouldn't even be able to touch you, really.)
BTW, I'm playing in DA, so is the autonomy augment even worth it? I would think pawns would be better for the distraction value.
Maybe, maybe not. If you have good ranger pawns its probably worth bringing them anyway... as I mentioned before, I'm using pawns but mostly because of the fact that I don't have Autonomy to even consider using at the moment if I had it, I'd use it (Not on DA though, probably would still use it.) because with the DR/Barbed Nails/Impact Augment, I keep Ur in pretty much constant stunlock anyway.