You need the legendary gear to complete it

can't recall but there was an achievement that needed epic or higher before
you can get crusader specific achievements even after the event ends anyways
for reference you need Legendary level 2 at minimum (hits 6 but somehow achievement will get 7)

Carlene, if you happen to get his relevant epic gear then you can temporarily disenchant something else to give him the necessary L, get the achievement, then return things to normal. Also, like most of the other event achievements, this can still be done after the event ends (though you will have to look up the requirements on the fan wikia since they will no longer be displayed in-game).

I assume it is the legendary eyepatch needed. I didn't manage to get an epic one, so I guess this achievement has to wait. Very disappointing that it is impossible to complete an event achievement during the event without spending money for more chests.