I need help for Deimos

I want to unlock the Helminth so I can do the nightwave challenge, but I'm getting my butt kicked. It seems that my heat sword does a ton of damage, while all my other weapons do nothing. So even though I'm running in there as Rhino with a ton of shields and health, I'm getting my butt handed to me because I'm usually in melee range.

I literally can't even complete a single quest given to me by Mother because I've just been overwhelmed by enemies.

So what tips do you guys have because I'm at a loss.