After many a try, I have found the solution that takes just 2 minutes and is practically guaranteed to work.
After completing the game and unlocking the 'infernal' difficulty setting load your save at or nearest to Malebolge 1. If you don't have savegame there, just play through it until you get to the first of the 10 trials.
The first trial gives you infiinite mana. As soon as the trial starts (after the description) go into the options menu and set the difficulty to infernal. This will make the enemies tougher, thus requiring more hits to kill them.
Be sure to have the Coin of Plutus relic equipped, to make your combo-time longer. I don't think it is 100% necessary, but better safe than sorry. Now, all you have to do is spam the Cross magic attack (LB+A) and make sure you hit all directions so you don't break the combo or accidentally get hit. 700+ combo guaranteed.
EDIT: I just realised i'm rehashing A Jugger's method, but it is good to note that you don't have to do it on the second playthrough, you can just load up an old save from the first playthrough.