I’d rather control one archidon and when they are not attacking, you can just shoot some arrows on the enemey speartons or archidons. Or call @r-mictal, he finished the game in insane mode. No upgrades achivement.

Try to take control of a spearton, to behind the giant, and attack.
When the giants about to attack, go behind him again and attack.
Repeat that process till the giant dies.