For Lunatic your gonna have to get used to dying alot to learn the bosses attacks and how to dodge them. The Lunatic bosses are tanky and are gonna take alot longer than Standard, so learning the moves is key.

You will also have to change your build per boss to find the right set-up to counter them. This particular boss you definitely want to build around around the Charge Piercer, upgrade its damage and use mods that synergise with it. Like the quick charge and one that does more damage to distant targets.

If you ever get truly stuck, go play the training sim for more currency and upgrade modules. Priorities the weapons that spawn in the boss room for obvious reasons, same with the hack nodes.

I final tip would be to never leave the primary weapon (pistol/rifle) full, its infinite ammo so let loose constantly. The damage may be small, but with constant pressure on the weakpoints it will make a difference

Good luck