I used a 150 element arcana mace and had no problems. you no longer have inquisitor mode, but you can learn the inquisitor skills again from you skill tab. The mobs in the quest die very easy and drop HP and MP pots when they die.
As for the last room, you don't have to kill the boss. just keep nuking him until the 2 pillars spawn. stop attacking and quickly talk to each pillar and that part of the quest will be done. The time limit is not about killing the boss or minions, but the time you have to talk to the pillars. the boss and minions will simply disappear. If you fail to talk to both pillars within the time limit, you will get kicked out and have to start over from the beginning again.