Follow a blue exclamation point in New Tuchanka to chat with Shaman Kortik.
Two powerful warriors are at each other's throats, and, long story short, he thinks making them fight a Fiend together should get them buddy-buddy again. The shaman even gives you a Fiend-lure, and a navpoint.
Place the lure in the indicated spot. You'll be immediately attacked by Wraiths.
Clear them out for the main event. It's only one Fiend, so just stay out of paw-range while dishing out damage. Fire attacks and hover-enhanced weapons work well.
When it's down (though not dead), return to the shaman. One warrior will agree to fight, but the other demurs. Follow your objective to find him outside.
He agrees to fight only if you fight too. So.
In the arena you'll be facing three fiends, and with these two jokers in place of your usual squadmates (though you can still summon constructs like Zap). As with lone fiends, stay out of range, pour on the fire, and use hover-enhanced weapons if you have them.
Here, try to keep moving in a slow, wide circle so that the Fiends clump behind you, rather than being able to flank you. When they finally go down, talk to the combatants to end the mission.