1. GET FRIENDS. You'll need a group of maybe 2-4 people if you don't find it easy. After all, that's why the Sigils are there... (thank goodness)
2. USE POTIONS. Potions fill up health and mana 100%. Use them when you really really need them.
3. GRAB THE WISPS. The red and blue wisps aren't there for nothing! Use them to fill up your health and mana gradually, so you won't have to overuse your potions.
And 4... DON'T GIVE UP. The old motto If you don't succeed, try try again is part of how you'll successfully finish. So just try your hardest, and sooner or later you'll be in MB, then MS, then DS, then CL.