There are a ton of videos on youtube, but there is one spot above ground near the center/left side of the lake where you can get a 6-7X multiplier. I figured that one out on my first playthrough and I take advantage of it when I pass by while playing. Otherwise, I haven't boosted yet and I'm at level 44.
1) 100% run on hardcore
2) 3% run on casual
3) Playthrough on insanity (I just past the exploding room and I have roughly 90% of the map and 60% of the collectibles)
"What you own is your own kingdom. What you do is your own glory. What you love is your own power. What you live is your own story. In your head is the answer let it guide you along. Let your heart be the anchor and the beat of your soul."
- Rush (Something for Nothing)