Invincible titan grotto boss - Act 2 Cruel

The titan boss "Zalmarath, the Colossus" is invincible to all the spells I use in one (seemingly random) position around the platform. (Either up or right. It changed across multiple attempts.)
I encountered this issue with an ice mage character: frost wall, ice nova, comet and frost bomb all fail to hit. (Also basic punch, not that I expected to beat it with this, but I wanted to check with at least one attack.)

Correction: it seems to happen at random every time the colossus moves.
Can be a little difficult to handle when it doesn't move for a long time while invincible.