Theres only one 'indestructible hangar' per town (the one at the edge, with the road connecting).Technically, this building does not belong to the town, as it is the ending point of the supply road connecting to the town - hence it can neither be nor does it need to be destroyed to capture the field). The other three grey hangar like buildings in town NEED TO BE DOWN.
Also, make sure you double check if all buildings are down (theres one small church wich can be overlooked very easy).
To bring troops you can either use a C47 (bomber) or an M3 (vehicle) carrying ten troopers. If you happen to have a CV close, theres also the option of an LVT-2.
With C47 drop the troops at minimum alt of 800 ft and a max speed of 200 mph. If i drop from the air i usually come in low and pull up straight over town while i drop (makes sure youre slow and high enough and the troops all land fairly close to the maproom). You can also land in town and drop when stopped, but you need to have an undamaged gear for troops to run.
M3/LVT: make sure youre as close as possible to the maproom, stop, open doors and let them run.
To get town down, either use a Lanc (needs some experience) and a 110 to pick up the leftover buildings, or, as Super suggested, two very heavy 110 and a C47/M3.
You can go in offline mode and try to capture fields on your own. Up from a field close to the enemies, flatten town with 110. Dont forget to destroy the two AA guns in the town (thats a neccessity). Bring troops.
For vehicle bases and ports without town: Just destroy all AA guns and drop troops.
In the Main Arena, since youre bound to have defense... practically all bets are off. Youd probably need to get the vehicle hangar down and have at least 2 or 3 fighters that keep uppers in check. Plus the guys working on town and a troop carrier. The more help you can get, the better.