NR totem is a must, as everyone has said. Another thing to remember is to re-drop it after the elemental disappears the first time. Don't pull the Colossus with Avenger's Shield. In fact, don't use it at all until the elemental. You can get initial aggro easily that way. For the second phase, when Avenger's Shield is on CD, pop your elemental.

Also, consider one of the nice BoE chest upgrades for your priest. Are you doing Chain Heal on the elemental phase? You may need to. Also, drop moonkin form and perhaps blow a tranquility if needed. Go nice and easy for the colossus, then balance nuking and healing for the elemental. I don't recommend moving to avoid the puddles, if you can help it. During the Colossus phase two are you doing Hymn of Hope? Also try dropping your Water Elemental as soon as phase two elemental starts. You will need the mana gen and the nukage!

Just some things to consider. There is a lot to manage with a mixed group. My Pally + Shaman group eats this guy up. Chain Heal x4 ftw!