When playing as the humans, one fatal mistake many people make is when they have maxed out all upgrades and population, they leave their field armory. When you get all the upgrades at a field armory, its useless, so scrap it and get a barracks if you don't already have one. Even if you have max population, Spartans don't take up any, so you can make some to get additional troops for free.
My strategy with Forge: Before upgrading your base size, fill it with supply pads and be sure to get plenty of resources with your Warthog. When you have your 5-slot base full, get turrets and then upgrade it. On the back slots, build 2 reactors and upgrade 1. This gives you enough tech to get all the Warthog upgrades, so you can make an army of Warthogs.
If done right, you should have 30 fully upgraded warthogs in 10 or 15 minutes. Optionally, get an extra base and fill it with supply pads except one slot, and put a field armory there.
I know these aren't cheats, but I think its still pretty helpful!
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