Protip: stop sucking.
Wait, you want something more helpful than that? Pfft.
I’ve done GB waaay too often (GRIM BATOL, FOREVER) although only once on heroic, but it doesn’t take a genius like myself to see that a pro bombing run makes a world of difference.
Look, someone even made a YouTube video. If you haven’t done the instance so often you know exactly what path you walk, don’t worry. YOU WILL.
The easiest way is to focus on one side at a time–say the left side on the way up, the right on the way back. Doing that should be able to visually trace your path and easily hit only the important groups. Don’t forget pats! Once you’ve done the quest ignore the troggs in the middle. You can firebomb from a surprising distance away, so you can keep plugging away at the groups for a bit instead of wasting time on mobs you never actually touch.