You can glyph mage armor to raise the resists a little. You might need to invest in some Motw drums to help the resists out.

I stand still against the pillars when I fight him and heal my ass off. It is important to burn the elemental fast and go slow on the colossus to catch up the healing. Moving will make the elemental phase take too long and create more purple stains.

For your priest get shields up, if you have time, and spam flash heal until the group is about 60-70% then group heal. Glyphing for the group heal helps because it puts a Hot on the group. When the colossus comes back group heal to full and start putting on shields again.

Another note for locks, park your imp away from the lock and other toons because the imp will get blobbed and if he is too close to the lock it will stack fast.