I just beat the taskmaster Djal in the lair of taskmaster Djal in Karanahn barracks but now it says I have to go do it again. It took me forever and it's really frustrating
If you fought Djal before you got the quest to do so, you have to go fight him again. There's no way around it.
If you poison the food beforehand, you'll only face Djal himself, without his guards.
Djal is a Storm wizard. If you're Storm also, you need to use Prisms. If you're Myth and don't have a shield against Storm spells, you might go to the Bazaar and buy some Storm Shields.
