*Don't read this if you don't want to make this game incredibly easy.*
So I've made such an efficient setup I thought I'd share. The game's become a bit boring though since I found it out, since I can't see when this setup will start to fail. I'm at $517 million, level 87. Most fights I barely take any damage, unless I get lazy / try to spice it up. If anyone has their own favorite setups, I'd be interested.
The Setup
(all maxed of course)
Bunker 1--3 Super Thumpers and 2 Power Shields.
Bunker 2--2 Air Supports, 1 Swarm Shot (doesn't matter which goes in this slot that much), 2 EMTs.
Bunker 3--4 Air Supports, 1 Time Delay.
Using this setup
The key to this setup is that it's nearly invincible, with tons of air support to demolish all over the map, and super thumpers to to clean up individual areas quickly/open strong.
Basically, set these three bunkers close together, close enough that bunker 1's power shield can cover all of them. Put the bunkers in such a spot that the thumpers are on a slope tilted towards a good number of enemy bunkers. This allows for thumpers to take out a large amount quickly. The angle's important since it allows easy aiming--when your arcing up high with your shots the angle's way too sensitive. A straight aim and you have more room for error / quick unloading.
So start with 2 spam salvos with the thumpers--this is likely to take out a couple towers. It's mostly just passing time though for the real action.
During this time the towers have been warming up their weapons. Now that they're likely to start firing on you, set first one EMT, then another, keeping the sky clear until you get your power shield's ready.
From here, keep the power shields up fulltime, using EMTs whenever there's a gap. This makes your towers basically invulnerable.
The rest is just up to style. The time delay should give you plenty of breathing room. The main action comes from the air strikes though. These constant barrages should quickly level much of the map while you keep yourself invulnerable. Obviously, don't use them when you see an nemy EMT go off. Just wait for the EMT to clear and spam.
Any towers not leveled by the air strikes should be taken out with the thumpers, or use the swarm shot for hard angles. Even if your base gets tilted such that the thumpers don't have a good aim, the air strikes should punish all the enemy bunkers eventually. Voila, easy win.
Progressing to this setup
The early levels my main focus is on using thumpers. Start with one tower with at least 3 thumpers, upgrading them equally. Position the thumpers as described above to quickly level the enemy.
Then create a bunker with another set of thumpers, or a couple thumpers and a spread shot. Basically, this is so you can have an easy aim on another area. The towers will be spread out at this point.
Eventually, upgrade some air support to do cleanup. I don't exactly remember how I got to this setup :P but it wasn't too hard.
Anyways, hope this was informative. Take care.