Ok, so I have a;
Maximus MG w/ Dual-Vent Muzzle Break & C-U-Gone Red Dot SIght
Kross SMG w/ D-Flex Red Dot Sight, Front Grip & Duct Tape Magazine
I had the Galactica SMG with the same Attachments as the Kross, but I changed to it. So can anyone give me some tips for what i should do?

Maybe try using the speed sling so you can switch to your secondary weapon faster? I prefer the Cab9+ext. mags myself

Dont have the speed sling. ive gotten everything from each challenge, except objective this. that ones hard