You guys pretty much covered it in awesome detail already. I'll add the strategy I posted over at GameFAQs here, too.
Posted 4/21/2011 8:08:18 PM by ball4376
Reldin The Mad Priest Boss Strategy (Act 5)
I'm playing my third character now (as a Mage) and being pure spellpower means I'm an egg amongst sledgehammers, for the most part. I use any means necessary when fighting a Boss in this game, whether it be hiding behind pillars, walls and tables or kiting the bad guy into a position where he's too stupid to move out of in order for me to unleash my fury.
My solo strategy here was to eliminate as many adds as possible while still having enough room to move around freely. It's difficult to kite bosses as a Mage until you can find something to work with (or around, in this case).
--FLESH GOLEM
The easiest way to deal with this fight is to pull the Flesh Golem back to the entrance of the Ritual Room (Northwest) and kite him in & out and around the pit.
--RELDIN (first form)
Reldin's first form will spawn back in the middle of the room where the Flesh Golem did. Go pull Reldin back to the pit like you did Flesh Golem.
--RELDIN (Ghost form)
Reldin's second form will spawn where his first form left off, so just rinse and repeat until mission accomplished.
There will still be a lot of adds during both Reldin fights as he summons them on the spot, but not nearly as many as in the huge open space where the fight begins and they are much easier to deal with near the entrance of the area since you have more control over the fight reducing blindside attacks by massive amounts of adds.
I'm sure it would take a bit of coordination with an online group to execute this strategy, because you can easily cut off other players in the pit by running up the stairs and around the railing. It might be something to consider, though.
On a side note, there is a Chest and a Book behind the portal that takes you to Howling Woods after the fight is over. Just select "NO" when it prompts you to "Enter Howling Woods?" and then make your way behind the portal.