My entry for the Rift Colossus Contest! Went with a Life based Colossus.
I imagine for attacks his jaws could crack open even more and swallow players by the dozens which would need to be rescued before digested. Attacking his sides near the stomach where the growth is very replenishing, so a high amount of dps inorder to break through the ever regrowing vines.
Fireflies that might land on players and explode.. poisonous pollen sifting down, slowing down players gradually placing a time limit on his defeat before they die.
His spikes could break off, turning in to treant like creatures which would have to be rounded up and AoE'd down, or tanked separately. In order to replenish those spikes, he could dig halfway in to the ground, taking nutrients from the soil. Players might find this a good time to attack him, but be careful as the spikes will pop back out as he replenishes his strengths and potentially impale or damage near by players.
His entwined tail could dig in to the dirt and attack people from below as well as just being a thorned flail.