Kill the single mob (74^^^) in the lower part of the zone.
Kill the 3 grouped mobs (74^^^ and 2 74^^) on the top. You will see a named mob there that is not attackable yet.
You will get another giant coming from below. Kill it, and you will get another giant coming from the top. You need to keep killing waves of these until the named becomes attackable.
Kill the named while you keep the still-spawning "normal" mobs under control. The normal mobs will keep spawning until you kill the named, so find a way to do it. In my case, I just kept the "normal" giant coming from below rooted while we killed the named upstairs.
The named gives no loot nor any AA exp, so don't entice people to help you with this for those reasons; they won't like you later.
This can be soloed, I did it with my wizard. Just have your Int up high and you roots well upgraded. And bring some heal potions.
ENSURE you do not hail or click the Avatar of Flames guy that spawns. He must float up to you and go through his speech uninterrupted in order for you to complete the quest. If you hail him he will stop and will not update your quest, even once he continues to float and talk later.
If you did stop him on accident, get the entire group to zone out via the lower doors, then once all out, you can zone back into Allim's Fate instance and do it over again. There is no lockout.