Magic crash details

Hex's Attack, Phys. Def., and Magic Def. will stack. Accuracy and Avoid buffs however don't

As for Magic Crash..

http://ellinforest.com/forum/showthread.php?t=10629

1) Yes, it works at zakum, but you have to use it BEFORE zakum uses physical defense up in order for it to work. This means that as soon as magic defense up ends, you should use magic crash.

2) Not sure, but it sounds like it should work.

As for Magic Crash working on Damage Reflection, a guildmate of mine tested this & it seemed to work the same way as it would on Damage Invincibility. You'd have to cast Magic Crash before it uses Invincibility/Reflection. I haven't tested this myself yet however.

My opinion though, I would pick Magic Crash to be level 20 and leave Elemental Resistance at 11 if in fact the above is confirmed true.