Holy Cow. You guys are awesome. Thanks to everyone for all of your replies and I should have given you guys more information.
The party consists of my 8th level Lupin ranger, an 8th level half-elf cleric, a level 7 dwarf fighter, a level 6 half-orc fighter, a 6th level dwarf fighter, a 6th level feral half-dragon fighter, a 6th level high elf fighter, and a 6th level human fighter.
We are playing 3.5 but we were playing with 2e rules for about a year. When we switched the DM converted the Revenants with his own homebrew format. They do have Regeneration and not Fast Healing, because in 2e they had Regeneration and the DM said he doesn't care that Undead aren't allowed to have Regeneration in 3.5. The problem isn't reducing them to 0 hitpoints, it is turning off their regeneration.
He introduced them in our 2e campaign because they were immune to turning and there was an undead encounter where our cleric destroyed a bunch of skeletons. He didn't like that so he found undead that the cleric couldn't destroy.
We tried acid and fire but they didn't work so we just hacked at them until we felt a sufficient amount of damage and hightailed it.