I find that I level my heroic levels fastest when I have monster boost and do floor 2 enslaver/cartographer runs. You're completing a few feats really fast when you do these runs as you pick up maps (every 15 or 20 maps is a feat complete), pets (every 3 pets is a feat complete) and you're breaking a lot of stuff, collecting a lot of crystals, etc.
I always watch a video when its available for a free boost, and if I get the monster boost, I'll stop whatever I'm doing and run floor 2 for the 5 minutes the boost lasts. If you don't mind spending a little $$, you can buy the monster boost too.