The best way to do this in solo is in mission 4: Original Sin. Choose the double lock on missile launcher for wave 3 & 4, and maybe the high damage sniper rifle if you feel brave for wave 5.
Wave 1: Mechanical Ants
Wave 2: Mechanical Ants, Mechanical Spiders
Just hop in the tank right at the start and wipe out all of the enemies, while blasting as many buildings as possible, this will make wave 5 much easier. Keep distance, so that the tank takes minimal damage.
Wave 3: Gunships
Hop out of the tank and use the missile launcher to take them out.
Wave 4: Gunships, Mechanical Spiders
You should be able to kill most of the spiders and gunships with the missile launcher. Hop in the tank to clear out the spiders if you're losing too much life, but don't bother trying to take out the gunships with the tank.
Wave 5: Mechanical Ants, Mechanical Spiders, Small Hectors
Ants, spiders, and 1 hector, spawn right at the beginning. Once a hector is dead, another spawns in. Some people have said that there are only 2 hectors, but when I did it, I swear there were 3. Use the tank to clear the ants and spiders while keeping distance from the hector. Keeping distance from the hector should be easier since you already knocked down most of the buildings in wave 1 & 2. As long as you keep driving, his plasma blast will always miss. If you're too close to him, his machine gun will do a lot of damage to the tank. Once the ants and spiders are gone you can focus on the hector. Keep some distance and you'll almost never get hit. Kill the hectors one after another until you're done.