Do not try to take all ship weapons at start. You will always confuse changing them. It is possible to complete first 1-5 leves using only plasma weapon. Open Dark Matter Bomb as soon as possible. Use it constantly on cooldown. When you will have enough technology points unlearn plasma tree and spend 3 points to Missiles and 2 to Bomb (the 3th point in bomb gives small bonus). So, use bombs attacking ground and missiles for air enemies.

Laser is hard to use it. Also, it does not gives so much advantage. You can use it only in three situations:

1. When 2-3 heavy mamonth tanks left.
2. For air bombers. Laser kills it in 6 seconds, missiles in 7-8 seconds, no so much deal
3. With bosses.

Your first free upgrade point should be spent on "Debris collector"
Use Debris collector as much as possible. If you use it often it could give you extra 3-7 towers per mission. Be aware, debris are dissappearing after ~20-30s.
Second point spend for "Core weapon". It will make defence much more easier.
Then upgrade Debris collector.
Then research Defensive drone.
Other upgrades are optional and don't gives huge bonuses.
Basic tower is useless. Try to upgrade it as soon as possible. Mass artillery for ground purposes until Laser comes. Try to place Artillery towers that way they could hit 2 different paths of enemies.
Use 75% Laser and 25% Artillery towers at the late games.
Always place towers for antiair waves. Sometimes 1 tower is enough to destroy all wave and you don't need to waste your time.
Use 2-3 Air Laser towers for Bombers and Heavy Helicopters waves. Air missiles for all other purposes. Stop using Anti Air Flak tower after researching better antiair towers.
Destroy buildings using Dark Matter bombs to increase the path.
You can destroy trees, trains and other stuff to clean the area for towers.
Don't destroy the bridges first. Sometimes they are necessary for buildings the longest path.
Enemies paths are always changing. Be aware and sell towers which are getting useless.
Sometimes, it is very useful to sell antiair tower during the wave when all air forces has been wiped out and build new ground towers.
Complete side missions. Upgrade points are necessary.
Destroy healers manually as soon as possible.
Dont waste your time on anti air enemy forces if you have anti air towers. Just move to them while attacking ground units.

There are two difficult missions: Russia and Japan...

For Russia, you need Anti Air Laser towers to deal with Heavy Helicopters.
Also, destroy the forest to the left to clean an area for towers. The most difficult wave is 5th. The 6 and 7 waves are boss waves and they are extremely easier.

For Japan:
Use 2-3 Anti Air Laser towers when you are fighting with boss. While Boss is sleeping, try to weak enemies ground waves destroying healers and the heavy tanks. 4-5 Laser towers at each paths should handle the rest. Use 3 Anti Air Laser towers for both air paths at the 2th wave. Use 4 Anti Air Laser Towers in the air path at the 3th wave. Ground forces for 3th wave are heavy. But it is much more easier to win it by killing both before the ground units comes to the core.

Don't destroy building at the Boss path before red boss line appear, because he will move through them then.