Don't go for hard, some levels are milimetrical. Why bother? Let play his game himself. Since we can't choose difficulty level. At least in console games, people can choose between easy, medium and hard levels. In kong, everything is at the desire of the creator. I do not understand why they are not adapting those difficulty because Hard badges should be based on time spend.
The force you to lose in some levels making you lose those designs. How can someone sell steel in shop and I making weapons out of steel and above and still losing in some battles in the final enemy??? pff
Most difficult levels are "Fight them on the beaches" and "Street Fight". You'll need a machine gun helicopter and snipe enemies before they see you without damaging the debris. I tried it many times and used to snipe to clear critical areas. Then just grab those citizen / VIP with the spy. In the street fight, for example, drive the mg helicopter inside stores and snipe both tanks to save those VIP. Bomb helicopter has a great potencial to clear areas all apart. Helicopters are better because they can shoot from any angle and pick floating stars.