Use focus target (Ctrl +F is the default hotkey), this allows you to keep track of enemy's casting even when you're targeting your tank.

Titan is an extremely scripted fight, meaning for each phase, he will follow 1 particular set of skills:
For example (NOT ACTUAL ROTATION) : Landslide -> whack tank few times -> Weight of the Land -> Landslide->whack tank few times->Weight of the Land.

If you're finding it hard to reactively dodge these attacks, you can try to pay attention to titan's skill rotation so that you know what skill is coming up next, and be ready proactively for it.