Nice little game you got there, I just beat it completely therefore can give a detailed review.
As many others mentioned, the sniper lvl (and also to an extend the rescue lvl) are much harder than the others. To kill the monster commander in the aniper lvl, the sniper himself has to kill dozens of other monsters first, that the base's maschine gun can't reach anymore. Sadly he takes forever to shoot, so the enemies come faster than he can kill, making the player pull him back multiple times until the monsters eventually come in a cheap order for a moment so he could keep up and try to get that one shot on the commander.
The rescue lvl consists of only using units, it took me a dozen tries to figure out it's best to collect 3 reinforcement groups at the base before advancing. Using the human robot units as tanks is also a must, or else the other units die too quickly, droping the damage output too much and getting the whole squad killed. Any reinforcements coming after that won't stand a chance against the enemies already at the base. When reaching the ship I'd have to select the 2 wounded VIPS immediately to pull them back to the base and using all my troops as shields on the front lines. If timed badly, the enemies can kill the two before they got behing my squad, and I can restart again.
Other than that the other levels where quite easy, I could've even finished the domination mode within 3 turns with only foot soldiers, but I wanted the 3 medals there.
And there comes the problem - the 3 domination medals won't unlock for me on the page here, although they do ingame. I just spent another 3h to build up those bases and max my energy income to lvl my frontlines and keep the last enemy base occupied, but even after reaching 100k energy, saving, and reloading the page, I won't get the medals.
If you'll ever notice this review here on that old game of yours, I hope you'll fix that bug (?), so I can unlock those last 3 medals ^-^
Besides that, good game, especially for its age. I'm not sure if the game quality standard back in the day was on that level, but I doubt it. It was frontpaged for a reason, so make sure it stays a good memory for old and new players ;)