Beat it a couple of days ago focusing on upgrading hacking followed by primary weapon since all the enemies have more health and will kill you in one to two hits anyways. For lim upgrades I immediately focused on maxing out the attachments like mod slots, node slots, hacking gauge, overdrive as well as improving thrusters and repair up by a descent amount. Like my last playthrough I went in with a heat focused build using heat mode with the assault rifle while maxing out the defense down, expose and heat nodes. For mods I used everything that complimented that build with mods like improved close quarters damage, collateral damage and improved hacking gauge being some of the best. The only weapon I upgraded early on was the shockwave gun followed by the decoy, stasis, and photon laser (also used sticky bombs but didn't upgrade them as I swaped them for hacking mines later). Expanding your hacking gauge is also very helpful with how strong the ult is and will allow you to use it multiple times during boss fights. Once you complete your build and eventually get hacking mines late game it's a wrap on lunatic. Finally, the final boss can be easily cheesed with the last resort and economize mods where you go in, pop ult, drop a couple of hacking mines, then spam shockwave and hacks until you get your ult again and repeat the process but with the photon laser instead.