or atleast buff the fans or mines

You start with a hefty metal income and not much energy. I usually find it worthwhile to sell off a mine to get turbines faster.

In the Guides section there's a couple here on Steam, and videos on YT of course.

The basic strategies are either rush out weapons quickly, like mass mortars or missiles, go for suppression with AP snipers/miniguns to take out their snipers so they can't fire missiles, and to tank up with lots of layers in front of your base while you get lasers or cannons.

I find the best way to beat the final mission is to get three mortars as quickly as possible, and then bombard the enemy into submission using the quick reload.

It depends on what level, in the default game i remember using overloaded missiles (I fired swarm and then large to avoid defenses shooting them down) due to the reload speed i was able to defeat it easily.