Making APC unload tanks is impossible but Badger Bomber parachuting tanks can be possible. LOL. Anyways, try to use this type of trigger like in the first ant mission where you have to defend it like what have you said, or the missions SCG08EA and SCG08EB. Then, also you might wanna take a look at the teamtypes and triggers then brainstorm by yourself and visualize the campaign you will create, like what I did to my custom RA1 mission "Assassinate the Traitor". That way, the campaign might result into something identical to those missions I suggested to you. But try not to make the teamtypes and triggers the same of course