What AP do I use for Xenon first?

I plan to use Pirate instead of Thief equips for end game (Tyrants and Fafnirs) since they're cheaper.

I heard on that Basil guide that LUK is first priority due to avoid but isn't it capped at 90% though?

What LeoDirk is saying is that you should go with your % equip, if you have any.
So for example if your gear happens to have 10% luk, 30% str, and 80% dex because you took it off your mechanic, then you'd want to be putting more points into dex since that would give you the biggest damage boost.