This contains spoilers : Endgame
If, like me, you found no n-ray weapons OR mods up to reading this however, this is for you;
Shortly after Phineas' lab is raided, making it the second to last mission in the game, you are tasked with going into his normally sealed off lab section after a brief fight.
In his room, Phins Phorce, a light machine gun with n-ray as its damage, is sitting plainly on a desk.
I put it on, saved my game, went to Tartarus, the games final location, and used it exclusively to kill the enemies in my way. The gun, with its 53x3 damage, always seemed to effect my enemies, and doing relatively high damage to organics, lead to a quick, easy 20 kills.
Note, I kept firing until they died, I didnt wait to see if n-way acted like poison. Supposedly its like Slag from Borderlands, only increasing damage to more n-ray as opposed to other damage types.
I hope this helps! I hunted for ages but either RNG was unkind, or I missed getting this earlier.
Happy hunting (>*_*)>