Don't use tank companions right now. They are horribly underpowered in the survivability department.
I've had no troubles at all on my Gunnery Commando. Currently L41, been using Elara Dorne the whole game. Spam Grav Round, use Full Auto on cooldown (Cryo Grenade beforehand gives you the 3rd tick), High Impact Bolt at 5-stack, and DO NOT go below 50% ammo under any circumstances. I suspect that's your problem. If you get to low ammo, you are totally screwed unless Reload is available. The ammo regen at low ammo levels is extremely punishing, to the point that I think it needs to be buffed honestly.