The Dark Wizards are one of the toughest boss fights in the game, in my opinion.
One attacks with ice, one with fire, and one with death. However, you can't usually identify which uses which until you've taken one of them out and they start using their regular attacks. All the same, it's best to get your armor and resistances up as much as you can.
When the fight kicks off, their usual tactic is to summon in their beast (which is tough to see making it tough to target so don't bother trying to kill it). While the beast is in play, none of the wizards will use any other attacks, so they're very vulnerable. Focus all your attacks on one of the wizards and keep pounding him until he falls. At this point, the beast will disappear leaving you with only 2 wizards left.
If you ever notice yourself starting to take damage, or if the ground starts to glow at your feet, then move to a different location to continue attacking. You really need to move around a lot to stay alive for long enough to bring them all down.
The dark wizards also have a curse on them, System Shock. This curse can only be cast at point blank range, and repels your characters away from the wizard as well as blinding them for a short time. This curse can seriously inhibit any fighter, and can also cause serious problems for any ranger (although as the curse can only be cast at point blank range, your rangers are generally safe from it). That, plus the elemental nature of their attacks, makes Spirit Embrace a very useful buff for your nature mage to cast before you go up against them. Also, don't be stingy with your powers. Use them, and use them as soon as they recharge. If a power needs to be targeted (Detonation, Circle of Frost, Thunderous Shot etc) then pause the game for a second to target it.
One last thing - make sure that your party is in mirror mode, so that you're only attacking one wizard at once. When one wizard dies, any others that are still standing suddenly go back to full health.
From cradle to coffin... Shall my wickedness... Be your passion