that is one of the most painful missions in the entire game. I remember doing it way back when with a PUG, and we basically had a few tanks kiting him in a circle while casters sat back and pummeled him with Meteor Shower and archers hit up some interrupts. We weren't really set up for it, but we figured out how to make it work, after most of us had accumulated so much DP. i think it took about an hour of kiting him, lol.
I recently went through it again with just my henchies on Hard Mode and it was pretty damn easy, haha. The 2 Mes, 2 Rit hero build (with Spirit Lord, I think) is pretty damn OP.
You can actually sit outside that gate, and try to flag your heroes down to take over a few of the altars. I think you can do it for all of them, but it's a bit difficult to see on the minimap. As long as you get a few without aggroing Shiro, you can get the others pretty quickly, then pull back up and out of the gate.
But Shiro has always been a pain in the ass. You can take a team of warriors and assassins in to him at the end of Factions, and using the celestial skills, kill him in about 6 seconds, or you can go in with what you think you would need--lots of interrupts, lots of spike damage, and he can wipe you pretty quickly.
One thing that really helps, if you're a tank warrior--Grenth's balance nec skill. As soon as spikes you with whatever that nasty skill is--I think Impossible Odds, or maybe the other one--you hit up Grenth's to reverse your health bar on him, then pound him as quickly as you can. Not sure if that works as well on his Nightfall version, though. I seem to remember Impossible Odds acting somewhat differently, or at least his frequency of use.