I have already got free 1500 above gold from game, but I wasted them, how can I earn more?

I have played this game from the start several times, due to my phone renewal. so I start to go again with Chinook to unlock more higher episode, use rental ship of possible, be patient not to use your gold until you can buy T5 flying fortress, then use it to earn more gold faster in custom mission, buy booster griffon $600, I am stuck in episode 10-2 for many days, I use rental Apache MK S, I use Shield, reload, emergency repair several times. I make it