Been loving this game!
This is great :D one tip for people stuck on the colossus. Upgrade your iron fortress till you get the "build attack tower" option. Then equip it as on of you're units, the Colossus' weakness is magic so have at least one withc and make sure you have spam units to help delay movements. When he starts moving put the attack tower between his legs and he'll try to pound it with his attack but he's so large he won't be able to hit it. This will buy you some time, keep spawning more and more though since the units behind it will try and destroy the tower. Eventually (in about 10 mins depending on your units) he will die. The best advantage of the map is that it's huge so you can keep doing this till he dies.