First you want to be on the map Spiral.
You want 2 Paladins and 2 Warthogs.
Paladins should have Ultimate Samson as their pilot. (Samson MIGHT work just not as well)
Warthogs should have Lexi as their pilot. (For the gun damage boost, others have it as well)
THE ONLY THINGS YOU SHOULD UPGRADE ON YOUR AIRMECHS IS THE DAMAGE AURA FOR THE PALADIN AND GUN DAMAGE ON THE WARTHOGS. If you upgrade anything else your not going to have enough credits to have enough units to win.
The units you want to use are the Gatty, Buster Tank, T99, and Ratchet
At the start, one paladin and one warthog go to the top-left base and the other pair to the bottom right.
The creeps will only be able to capture 2 of the bases, so make sure you pick them up at the fortress after they have the first two and drop ten off at the other 2.
You want to build about 10 Gattys at each base, as close as you can.
Then you build between 3-5 Ratchets at each spot.
Then you build 6 T99s and place them 3 on each side of your fortress. Try to have this done on wave 4. You can then ignore the air units for the rest of the time.
After you have all those setup, build Buster Tanks exclusively to try to reach the unit cap. Ideally, you want to keep the Buster Tanks as close as possible so the Ratchets can heal any all of them. If you notice that a Ratchet has died, try to replace it.
The Paladin and Warthog on each side should spend the entire wave shooting the enemies from the ground while standing in your army of Gattys/Buster Tanks, while building more Buster Tanks. ONLY GO TO GET THE BUSTER TANKS WHEN THE WAVE IS OVER OR YOUR BUILD QUEUE IS FULL. You don't want to go get a Buster Tank and come back to realize that 3 of them died while you were away.
If one side is finishing off their enemies faster than the other it might be a good idea for everyone to drop their Buster Tanks on the side that needs them. For example, if one side is seeing quite a few of their units being destroyed halfway through the wave, they should tell the other side that they need to be replenished so that by the end of the wave everyone can drop 2-3 Buster Tanks each. VERY IMPORTANT TO ALSO REPLACE THE RATCHETS IF THIS HAPPENS. Wave 9 can be a bit of a doozy, but if you are less than 50 from the unit cap you should be good.
This method also works for Solo and 2 Player Survival, with 4 Player Survival being the hardest by far. Practice this method in Solo before you try it in 2/4 Player so you know what has to be done. In 2 Player, both players should Paladins with the damage aura.