Completed the first mission with auto-cannon today from the first try. Little sneaky bastard tactics also helped. I guess you should really take more appropriate weapons for each mission and change your tactics depending on the situation. Also helicopter felt near me and and damaged my armor. That is a really nice touch here. And I noticed at last that buildings are not just sprites - the perspective changes a little when the camera moves so you can see they have a volume. Really cool.
P.S. BTW - tested the game both on my pretty powerful gaming PC with Windows 10 and on my old laptop (ASUS N53TK). Runs fine on both of them.