I got it on normal mode using Faye as a Gold Knight, make sure that you promote once she makes it to level 20, it takes a ton of grinding in the Thieve's Shrine, but when you go into battle use Clair to bait Slayde. Then get rid of the enemies in the path to Desaix except Slayde. Once Desaix is at low HP choke off his route to the healing tiles and spam Armour Breaker.