A few tips:
-Quality beats quantity. Prioritize upgrading your existing factories over building new ones. Out of those upgrades, I recommend prioritizing range and rate of fire.
-The second level is easier than the first. The Dark Cultoids only use gimmicky bots and you should be able to defeat them with only a bit of effort. Once you've done that, go back and challenge the first level - the Speed Demonoids are very useful at creating an offense that doesn't melt the moment a thunderstorm happens.
-Having trouble with the tanks on the fourth level? Chain Bots are brutally effective against them, destroying them with a single touch. Just make sure to pair the Chain Bots with something that can distract the tanks, like Turtle Cruisers or Speed Demonoids.
-The best defense is a good offense. Don't bother with the Sniper Bots, they're expensive and 90% of the time they do nothing and then get destroyed by an EMP.
-Don't give up! If your wall is being damaged early in the game, it doesn't necessarily mean you're screwed. Keep upgrading your factories and more often than not the tide starts turning in your favor.