I use Gauss Barrel pretty much exclusively except for certain parts. For example, in Turn 3 of Alexander, I'm in GB for the first two phases. When he switches to Tornado mode and adds start popping, I drop Gauss in favor of being able to run around and kill the adds quicker. The instant cast is so much better.

Also found a use for Mind Rend-- helps a bit with Cascade, though the minute and a half cooldown means I can only really use it on one.