So my friends and I used a new cheese spot for Sepiks on the Nightfall that worked really well. Up the stairs on the left from where you start the fight, there are two sets of windows with some panels missing or broken. If you get a small running start you can jump through them into a "secret room." Like with the Valus glitch, no one can hit you in this room. From there, you can shoot out of the missing window panes if you have a Sniper with Armor Penetration, like, say, the Vendor LDR. (It does have to go through the open panes, and it does have to be ArPen.) Take Ice Breaker with you so you can regen ammo, but with three people, Sepiks went down pretty fast.
There's a video showing where the room is here. The guy doesn't use it as a cheese spot, just a place to get away when his health drops.
Hope this helps someone. I tried soloing it on my third character after my friends were done, but I couldn't get past the waves by myself.