You must be at least level 45 to start this quest.
Double-click the pillar again to open the instance [Oracle Blocks].
Here is a map of the dungeon with monster placings:

The first group of monsters you will encounter moves around the circle in the direction of the arrow.
Once you clear those 4 groups, proceed to clear the 6 rooms one group at a time.
If you don't defeat all the monsters before attacking Ares, they will all come out of their rooms and mob you.
When Ares' HP is around 10-20% he can summon a swarm of Bronze Howlers, to prevent having to battle these make sure you lure ares out of the inner circle first (preferrably closer to one of the previously mob-infested rooms).