User blog comment:B214/Chapter 501: Mari and Randi/@comment-24546844-20160912135626/@comment-29693708-20160912173109

actually the nakagami armour as a few weaknesses; since it is enchanted with the power to nullify magic, non magical attacks still work, rules of the arena could undo the requip, bypassing the armours magic; if you had a weapon that could match the space slicing of the halberd and you are good enough at wielding it (like ikaruga and her sword), you could wait until erza is out of magic (which wouldn't be very long given the armours high energy consumption rate); nullification magic and disenchanting the armour could also work.