Stop moving. It's an awful tactic and not needed at all. The puddle does like 500 damage per second to people with zero resistance. toons with even just 15k life should be fine.

Across from where the colossus spawns, put your guys with their backs against the wall, spread out. With their backs against the wall, when the elemental surges them, they won't get knocked around. If you get multiple puddles on top of each other is when the problems start. That's why you never move your guys and start them spread out.

Pull the colossus. Tank him a bit away from your group. Make sure he is facing away so that as soon as he spits out the guy, you have a second to pick him up. Save a taunt or something for this.

During the puddle phase, simply HIT YOUR AOE HEAL ON EVERY SINGLE COOLDOWN. EVERY single one. Put it on a button next to your spam button or in your healing rotation and smash it. I only say every cooldown because you're new to this, and keeping your guys topped off will make you less likely to lose your mind and do something silly. I personally hit wild growth once now, and then forget about it.

In short, stop moving. It's silly. You are taking so much damage because you seem to have tank aggro problems to start, so work on that. Also make sure your toons have their back against that railing/wall and that they are a bit spread out. Do not make this fight more difficult than it is. It is simply a tank and spank with a completely negligible aoe and some aggro wipes. Concentrate on your tanking.