Enjoyed the game immensely, and eagerly await the next update. That I think if you are going to have the pricing scheme as you do (with fighters so blasted expensive), I would make them have a much greater income at an equivalent price, than the other units. I'd site Ad Cap as a good place to look as it has a similar pricing scheme as this, but the item that has the highest price remains useful all game, while the fighters really don't, at least in my eyes.
Lvl 8 is easy just get all atrillery and torpedo batteries then go destroyer crazy like fifty. These all will die then attack with carrier destroyers and battle ships galore repeat but make more for flagship
For all you who think this is hard just do the first level a bunch of times get everything and swarm the second level and so on with kick but level 10 five star units. build about five airpots three or four war plane 2 and a war plane 1 swarm the guy with tank s