What I have been doing with my team, to fight the colossus in his elemental phase is the following:

1. Position all 3 dpser on the walls of the circled room (the one befor the boss, where the elementals and the 2 'statues' were), with a little bit room between them, but not as much.
(the 'on the walls' is, so that the elemental stays before them, when it charges arround and the ppols don't add up)
2. position the priest before the dpsers so, that a 'holy nova' heals all dps (and the tank)
3. position the tank infront of the priest
4. use an nature-resitance thing ( for me its my hunter, for you your shaman)

with this constelation, in the elemental phase, i simply stay where i'am, tank with my paly, and dps with my toons, while my disc-priest spams holy-nova like he's crazy :)