Complete the Bomb Them Again! quest in under 2 minutes 15 seconds while not in a group.
It's been a while since I did this achievement, but if it's not fixed: You can cast two bombs on the same cooldown, which makes this achievement very easy. Find 2 targets very close to each other and cast very quickly on both targets. The attacks that dismount you are easily avoided and you're already on to your next targets until they get in your range.