To accelerate the level up, you need two things :
- A good magnum (L.hawk has a better firerate) or any good weapon you like.
- The "level adjuster" custom part (you can get it in any stage in ABYSS).
Set your level to 27 (or more if you want an easier life), attach 3 "EXP Up" perk (one for each weapon) and run in the first level in ABYSS like this :