I finally unlocked it against a Sobek in Arena. Super easy to stack marks on him when he ults.
Mode: Arena
My general build: Reinforced Shoes (Now removed so start with a sustain item or an item with CDR), Breastplate of Valor, Chrono's Pendant, Spear of Desolation, (Then two items that helped with sustainability, such a tanky item like one of the cloaks)
Tactics: You want as much CDR as possible so this build allows for CDR to be over caped via item passives to allow for the maximum CDR possible. Next, you will need to be able to stay in the fight to spam your abilities and mark enemies with all five marks before they disappear. Your best bet is to focus on one enemy and remember you must hit that enemy with all 3 versions on Le Fays 1 ability, her 2 and 3 to build all 5 marks and then absorb them with her Ult.
Recommended target: Tanks { Especially Sobek because of this Ult form allowing for easy marking)
Hope this helps!
Also check out r/SmiteArena