Hey man, just try doing this:
lvl 1 : build 1 grenade turret, upgrade it once and put it at a corner. If you have stragglers, add a single gun turret near the end of the map.
eventually when you can, buy another gren turret, put it on the opposite side of the first gren turret. upgrade both as much as you can till you fight the first boss ( the cat thing, i think ). Once you beat the cat, use the money to buy a grenade centre thing.
Once you have enough, sell your single gun turret and buy a missle turret and put it just out of range of the grenade turrets, and upgrade it as much as you can. Upgrade the grens also.
Those three turrets will get you pretty far. Along the way you can add another missle turret if you’d like, and keep trying to build the other research centres. Once you get the lazor cannon up, it’s easy mode from there on out.
BTW this is a strategy for “Hard” maps, I’m sure the easier maps can be done… easier.