There is an easier way to do this achievement.
You need to buy a bunch of 40 gold coins potions, start a solo mission, press LB (taunt) at the very start, run on the first enemies, use a rage/focus potion then press RB and kill them with jupiter god power. When you have killed the first enemies, instant press start and quit the game. Begin a new game, taunt as soon as possible and when attacking you will see you have keep your combo on. I won more than 2.410.000 XP in one combo of x2069. I level up by 48. (125 to 173)
The key is to play safe, XP gain increase exponentially. Using focus potion make it easier and quicker. Good gear can help you too.
Warning: don't go on the dashboard during loading, you will lose your combo.
If your french there the solution in french:http://www.jeuxvideo.com/forums/1-24300-75061-1-0-1-0-s....htm
My videos are dead but you can find some people on Youtube using more or less my technic: