the game isn't that hard, I finished easily the medium campaign on my first try.
THE YELLOW LINE ONLY MOVE WHEN YOU UPGRADE YOUR PLANET (i think it depends on the total cost of your weapons).
=> wait as long as possible before upgrading.
just have 4 shields and 4 weapons as soon as possible start with machine guns than replace the 4 machine guns with 4 plasma with at least 1 dmg upgrade for each, (I did half of the campaign with 4 unupgraded shields and 4 plasma guns (2 dmg upgrades), and each a battle was worth 1600 at the end).
you should be able to block almost everything with your shields. When you start to have trouble, upgrade the cooldown of the shields
and when 1 shield isn't enough to block the shots from the enemy machineguns, upgrade the damage blocked (once is enough).
when you're rich enough, replace your plasma guns with missiles launcher (reduce the cooldown to 6second to be effective). the rest shouldn't be too hard.
Thanks for playing and sharing your strategy! This might be a useful read for everybody who struggles with beating the game. You are right, the yellow line moves with the "total value" of your planet, (max value of all weapons and upgrades purchased so far).
in the level 14 of the alien campaign, the enemies always suicide before I can kill them and it doesn'd count in deathmatch, it would be better if a suicide counted as a kill or at least as a -1 pt for his team (it happens often 5-10 times and sometimes more than that in the deathmatch)
did it in 18 days ( yes , it's hard)
to earn many money in each flight :
use your rocket as soon as you leave the ramp and go up ( your speed must be almost in the redzone but not in the red zone , or you'll loose too much speed ) , and when you don't have anymore fuel, go down to keep the same speed (still almost in the redzone )
be careful you must not touch the water , when you are at almost 0 altitude , fly horizontally until you just fall in the water
i tried a lot of techniques , and this one works very well to earn money and to have the max distance and time achievements ( when fully upgraded , i reached 7804 feet on the last ramp , and reached 6131feet on the first ramp)
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Thanks for the perfect score! =)