AA is fine as is. It's only difficult on the lower levels since you don't have the time to build up your forces to level 3 on some missions and add support towers. Make sure to put stun towers in your line of AA to slow down planes. All the missions are doable.
I find the irradiator and tesla coils much more useless. By the time tesla coils unlock I don't even need them.
Also cannons + stun guns take out jammers(the tanks that turn off your towers). Air strike helps if needed.
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