You can parry magic damage as successfully as you can kite vortexer in A6S without getting hit
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If the spot for the 5th crossclass belongs to a different skill than the 2 you've listed it's completely fine to say that. You can't just go making up arbitrary rules like that.
Also seconding feint as a sure 5th: raging strikes, b4b, invigorate, hawk's eye, feint. With 6th place being quelling and a distant last being keen flurry. If you *need* quelling because you have a bad tank that's a different story, but doesn't make it a better skill.
Well, I've used Keen Flurry a few times in the past on BRD... but its very situational.
It was most useful to help reduce incoming damage from Earthshakers on T13, and it also worked on the missiles in A1 Savage. It could probably be OK for PVP too!
But, Quelling Strikes is good to have in most encounters, so that wins overall.
Some magic attacks can be parried even though it does not make sense. The game has exceptions for certain attacks. In T5, Twintania's fireball attack can be parried even though it is considered a magic attack. T12, Phoenix's Revelation is a magic attack and it can be parried too if I recall correctly.