For some reason I can't find the option to edit my original post again, but I just helped another forumer (Gidlee) all the way from killing the level bosses to taking out the trio. What was freakishly effective? A zealot with moderate gear and a Doom Leap barb.
I'll explain:
My zealot was my first character when I started playing again, so like in all those situations my stat points weren't too well distributed in order to accommodate for poor quality gear. However, he did have a point in salvation and zeal maxed, along with it's synergies and fanatacism. It was pretty much outfitted with the gear from Pougee's Dclone killer build. When alone with the bosses, the char is pretty much unstoppable thanks to life tap + a fast zeal, and with tons of crushing blow and deadly strike it whittled down the bosses pretty quickly.
The barb's skills focused on WW and BO, two crucial skills for obvious reasons. He sported 2x Doom Zerker axes, giving him a nice high level fanatacism aura, which then overflowed onto the zealot.
How did this work?
It was pretty simple. We both had life tap wands on the switch, so was soon as we encountered any major groups of summoned creatures we'd tap them, then zeal/ww them to death. However, leap is a nasty little trick that allowed my zealer to dominate all the bosses with relative ease, including Meph. With his 2x Doom's level 18 fanat (stacked), I got a major IAS boost and went using salvation (1 point wonder) instead. With mercs using defiance and my salvation, Meph's conviction did nothing to us.
Then it was just a matter of him leaping around the bosses, knocking all the monsters away while I zealed away and let my crushing blow and deadly strike do the work. Just 2 chars dominating the trio!