NeuroVoider

This along with the other completion achievements, "Finish the game on X mode", are stackable. You only have to do it once on the highest difficulty setting. I would recommend being able to complete Arcade difficulty first so that you get a handle of how the game works. Don't bother trying to loop the game on any other difficulty but Voider if you plan on achievement completion. The game isn't really that much harder on the other 2 difficulties, Arcade can be a challenge enough until you get a good build.

Very Important: After you defeat the 2 forms of the final boss, you must spare FAT.32. Don't shoot after the second form is down. If you accidently shoot him, then the game will end and you will not be able to loop the game twice and will lose all your progress.

Tips
I used the rampage class with Self Repair for my runs. High DPS is required as you don't want the boss fights lasting too long. Healing is also a must. When picking levels, large size stages with elites will drop more items. Special stages with super elites will always drop a group of items. Picking larger stages will net you better equipment but take longer, you can find your own balance based on what kind of drops you are getting. As far as equipment goes, later stages will always drop consecutive better gear. You will always be scrapping and boosting new equipment all the way up to the last boss on the second loop. Once you get a good setup, I basically use rampage to kill everything and maybe my low EP weapon with a machine gun to pick off the smaller enemies.

Vision
Gold colored is always the best for the drops you get on a specific stage. I chose recharge and hp repair skills priority over EP. Sometimes you will go many stages without a compatible gold part. If you are close to another boss fight, it's sometimes best to boost a different color part if you have not updated a part recently.

Core
The most important thing you are looking for is EP/s, the higher the better. This allows you to fire more shots without over heating. The ideal is to be able to hold the rampage button (RB) and either take many shots to overheat or not overheat at all. Overheating with a single rampage shot means your EP/s is too low and/or weapons are too high.

Transport
Just boost the best ones you come up with. Good overheat ability can help.

Gun L/R
This along with the core are the most important builds in the game. I chose to keep a very low EP/s weapon on one slot (can even go as low as 40s). On the second slot is the highest DPS weapon I can find that, in order of ranking (lowest first) is frag, missile(multiple/forward firing), then nuke. Having a high DPS nuke that never overheats while using rampage can make the game very easy. As you get better cores, you can slowly add more EP/s until you are barely overheating with rampage. If I have a very good nuke, sometimes I will upgrade my low EP weapon a bit more if I can.

There is probably other viable builds, but I never tried them. I never used melee weapons, even with double the DPS of a nuke, I could never do any good damage with them. Other weapons that shoot in all directions are a waste of energy because you want to be able to focus all your fire in one direction for the boss fights and harder enemies.