You've got to practice it, really, it's hard to describe the timing. If they DON'T have Pneuma up, just hide during the cast time. If they do, you need to hide so that you're still in hide during the split second between when your animation finishes (the hide animation) and Pneuma pulls you back out.
It's not so bad if you practice a lot. An easy way to practice in town is to have an HP friend cast Pneuma and cast various levels of Assumptio on you and try to hide-dodge the buff itself.
