Clancy, you have to win the battle after you have killed the monsters with the trap. Also, the quest says, you need to kill 30 giants in a battle. Not quite that easy, if you are at D12 for now, I would think.
To kill a beacon with a Gem bomb, it will need to be level 6 to eight to have enough attack. Or a lot of smaller level ones. Continue the journey through the labyrinth, there are way better levels for this one ahead :)
Thanks so much for the quests. I lost a lot of interest, once replaying all levels turned into creating a mana farm and having it run as long as possible. I would consider the Red Mist quest as hard, though. I didn't have any trouble with the first pylon level, but having 300 Monsters on screen after a 300-chain-kill requires some serious setup.
Cheers