Okay people, here is how to do it:
1. Get a warlock (lol)
2. Summon an Imp (teach him Blood Pact and Fire Shield, if it does not have it already)
3. Let the Imp cast Blood Pact on you, turn it on Auto-Cast also
4. Click on your self, and click the Imp's Fire Shield spell, it'll cast Fire shield on you
5. Dismiss Imp, It'll remove Blood Pact from you, but Fire Shield will remain on you
6. Summon another Imp, and while summoning Remove Fire Shield from yourself
7. Repeat from step 3.
8. Enjoy!
I got around 60k mana yesterday using this exploit. Tested and working on W0WSc@P3.
EDIT.: I'll try to post some screenies here soon.
Okay, some screenshots: