This Guide is more a simple one, not much Wall of Text:

I just won on brutal, so i know the game.

1. Stay alive (Meta): Gurdian Command, Aegis (Shield), Shield Surge
- extra tipp: build your Aegis on the right side, otherwise the Overlord will destroy it (you maybe lose it in other Missions, but this doesnt matter)
2. Stay alive (additional my personal Favorites): Cyrohawk (nice Anti-Air Secondary Weapon), Talos (vs. Rockets)

3. Make some Damage (Meta): Cyclone Control (its allways the best!)
4. Make some Damage (additional my personal Favorites): Punisher, Spectre Control (it only performs well with the Carpet Bomb Resarch), Ranger Command (many Resarchs needed, but with Ligthning Arrow and Wildfire Arrow in the end it performs also well),

5. I also used Time Warp, but i'm not sure it really helped that much
6. In hard Missions i sometimes used the free Rebells without any Tech as well

The key of winning is the Meta: Gurdian Command, Aegis+Shield Surge and Cyclone Control. All other things can be experimental and by personal Favorites. You can try Rebells more or other Primary/Secondary Weapons. Don't trust too much in Towers.

Dont use Sentinels, they are too expensive.

Starting Build Order:

1-2 Guardian Command
1-2 Cyclone Control
Aegis (if you got the right ressource, otherwise build Ranger Command, Talos or resarch your Punisher
Dont forget Research at all (have some blue Cores? -> build somthing, dont have blue Cores, resarch something)

Maybe you will lose some Missions, but this doesn't matter (you get the Resources and that's it)

End Build

6 Gurdian Command
3 Ranger Command
6 Talos
9 Spectre Control
Rest = Cyclone Control

--- English is not my main language, sorry if i wrote some ♥♥♥♥♥♥♥♥ ---