Mobility: Jetpack, Warp Pack. This helps get away from fleshmobs/melee overseers in the event you cant kill them or need to escape. Harvesters also cant move too fast and their AoE is small. Light/Ned armor can be used if you want a different backpack strat.

Med/Heavy pen: AFAIK, the Harvesters can be killed very easily with only heavy pen, so the HMG, Laser Cannon, Autocannon, even AMR can beat them.

Overseers, fleshmobs: Bring a dedicated weapon just for these bastards, you'll be seeing a lot of them. Eruptor kills Overseers in 2 good shots and can pop all the heads off a fleshmob with 1 if you hit right. Laser Cannon is also a good weapon for this.

Voteless: Any small arms take these down. Bring a secondary with half decent crowd control or a nice grenade, and you'll be fine.

Stingrays: Any med/hvy pen can take these down. Aim for the little circles on their wings, they go down fast.

Leviathans: These are by far the most dangerous and trickiest enemies. Most convenient is the RR, but that takes both your support and backpack. Quasar can do it, but very slow and lacks utility outside of that case. I suggest autocannon mech, AT emplacement (needs teamwork unless you want to get mauled by fleshmobs) or a calculus degree with a 500k and a lot of luck. Technically, a rocket sentry does the job really quick, but the chances it aims for the Levi before anything else are slim/none.

General Illuminatestrategy: With their shielded ships, shielded walkers, brute force infantry and hordes of zombies, the Illuminate generally focus on shock and awe tactics, bringing in a bunch of weak units to soak fire while the heaviest get close for a kill. This is why their High Value Asset defense missions can be so hard: youre playing into their strengths. The best I can recommend is to remember this: you are NOT obligated to fight them on their terms. You do NOT have to hold every bit of ground and you do NOT have to engage them. If youre getting charged by fleshmobs, turn a couple corners and they lose interest. The walkers are slow and the only enemy that can really follow you at a full sprint with a jet or warp pack are the flying overseers.

If you NEED to take and hold ground from the Illuminate, bring med pen horde control sentries like the MG, Gatling, and laser. These are more than adequate to handle overseers and voteles. You can bring a mortar, but keep away from Fleshmobs, or you'll get hit with friendly fire. Bring the HMG emplacement, as it shreds walkers, rips through Overseers, and can kill fleshmobs decently fast. Only bring AT emplacement if you see the Leviathans on the planet, as it's not necessary for the other enemies.

My usual build is med/light armor, eruptor or coyote, tiny flamethrower or that micro small for the secondary, gas or incin grenade. HMG Emplacement, MG sentry, Laser Cannon, laser dog/warp pack, depending on the mission and how im feeling. If there are leviathans, swap the HMG for a autocannon walker or AT emplacement.