For the Colossus, the strategy I used it to use Iron Fortress with omega seal, it kills all of the enemy ground units with it's attack. Use a couple of fast melee units to keep the Colossus busy (it helps to equip them with the flaming chariot you get from trial 4) and the rest ranged units (I use centaurs and elves). If the Colossus' attack gets within range of your Iron Fortress, use the wall spell to push him back a bit, and use the regen spell to heal your Iron Fortress if it is low on health.
@Worgs if it was actually level 1 that they had Barrett at then they are definitely a hacker (unless it's glitch), because you can buy up to +7 accuracy and Barrett needs 10 accuracy. +1 from level skill point + 7 from artifacts = 8 < 10
 "Epic War 5"){kind=icon}
 "Waraxe"){kind=icon}
 "TDP4 Team Battle"){kind=icon}