Ez-mode Guide:
1.) Save, buy truck 1-2
2.) Buy truck 4-2
3.) Buy 4-1, keep truck 1-2 equipped.
4.) Upgrade the weapons on level 4 trucks, fitting in a truck 1-2 ability upgrade. Use this when you get overwhelmed
5.) Continue to upgrade level 4 trucks, upgrading abilities of 4-1 and 1-2 as needed.
6.) Easily beat the game using 1-2's slow motion ability.
I won with 3 UAVs and 4 slow motions.
Easy win:
buy two Yellow guys immediately, then upgrade them one at a time to Chronos
Then buy a corpse and upgrade to death eater
Then buy two more yellow, upgrade to striker one at a time
Make the death eater a entropy alien
strikers->ultimas
Chronos->Chakra
Then max the chakra, ultima, entropy one at a time.
This build will get you to the last level no problem, with no deaths. I'll leave the final boss for you to figure out, you'll have plenty of cash to swap your build at this point