How to get gunship battle episode 7 mission 5?

Purchase Sea Knight. You should clear that mission easily.

Well the super fortress and flying fortress are good choices as you would not have to stop and kill your targets one by one(there is also danger of deadly missiles and fighters) like a helicopter,it will kill all the enemy and buildings and fighters in the sky and ground automatically and u do not need to kill each Target by your own and the automatic cannons's ammunition does not decrease from your money(does not cost ammo cost)