I got a big issue in the dragon battle. The enemy archers were deployed in a spot surrounded by mountain, so it's basically impossible so kill them, since I got limited ammo (and the archers insist to attack the dragon)
Well, I'm full of health in lvl 26 and stuck in an arena... I killed everybody but the door just won't open for me... I guess I'll have to lose my progress today...
To beat Colossus, try picking populous type of units and keep sending'em to keep colossus from reaching you hero. Use a lot of agressive magic against it. It works, but you have to keep a good synchronism or speed at 1x.
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