I just finished path 3 and I have to say the mechanics for Colossus Rumblus are glitchy at best.
When my party went to engage Colossus Rumblus and when the Rocks started to fall, Gast would either never put up the shield or put it up way too late to prevent the damage. To me, Gast should run in and just throw up the shield, not wait until after the first rock fall and everyone is down.
Secondly, Gast died way too easily. He needs a few more Hit points or some stronger damage mitigation.