As mentioned above, you must do a pre-quest from the same quest giver to get this quest. The first of this series is called "Disrupt their Reinforcements" and it involves destroying the two green rocket launchers just north-west of the quest giver. You must kill the demons guarding them for their green drops to use to charge the detonation device. That quest gives around 10k exp and similar rewards. I barely made it without popping to 60 (1 bubble left, started at 1/2 exp or just over). [See screenie above for quest description]