I would say this is an exploit, cause surely this isnt the way Blizz intended those achievements to be done.
There are 2 achievements for this fight, one is to kill his drake in 1 pass, second is to kill him within 3 minutes. First one is easy, second one can be tricky if you lack DPS... NOT ANYMORE
So here is what to do:
1) Start the boss event
2) Kill the spawning mobs till your party gets 5 harpoons (or more if you want to be safe)
3) Run back out to the hallway where the previous boss was, the event should reset (or you can die and corpserun, same effect)
Now you have a reseted event and 5 harpoons in your bags. You can easily charge through the mobs killing just 2 packs + 1 spawning pack, use the harpoons when he is in position (first achievement completed) and you have alot of time to kill him before the time for second achievement expires