It is feasible to do solo, though only if you don't try to engage in a straight up fight. That's where the distractor beacons come in handy. They are good for stealthing, but they are also good when the shit hits the fan. Start running till you get a little ahead of them, then toss it in the opposite direction and once you break their sight lines most of them will get distracted and the heat will die down. That mostly works best on the illuminate though as their units are slow enough that you can outrun most of them. Standing and fighting if it is not required for an objective will only draw more of them to you as each time a scout spots you it will get worse and worse.
A good loadout is a primary weapon that can kill medium armored enemies, armored piercing and/or stun are good attributes lasers also work decently against armor. A stratagem and/or secondary weapon that is a tank buster like a railshot airstrike, a recoiless rifle, demolisher charge, vindicator air strike (fully upgraded), EAT-17 RPGs, obliterater grenade launcher, etc. And some kind of area denial stratagem for the shuttle call like a mine field (when fully upgraded it is also anti-tank), stasis field airstrike, naplam, razor wire, stun mines, tesla tower, one of the two mechs with anti-tank L2 weapons (walker and lumbrer), etc. If you are using a weapon with ammo, it's a good idea to work in a fully upgraded resupply pod.
One of the worst objectives to try and do in a higher level mission is ordinance disposal as not only does it take forever to locate the mines, but while one of you is looking for them, they aren't able to help you kill enemies. If possible try to tackle harder hold this point objectives (like the capture points which you have to keep it clear of enemies) first.
When soloing a lvl 11 mission yesterday for samples I made the mistake of saving a capture point for last and i almost ran out of distractor beacons as I had to keep running away and dropping beacons to draw them away since you can't clear a capture point of enemies fast enough at that difficulty level by yourself.
I'll be busy with other stuff this afternoon but I can help this evening if we want to tackle tougher missions. The HAV (heavy assault vehicle) with at least one repair gun user will also help a lot of with high level missions for a coordinated team as its main gun kills most tanks in one hit.