Hey there! The Myrmidon's Shield Wall exploits an issue with how Blizzard programmed Devotion Aura. If you remove the Devotion Aura buff from a Devotion Aura-based ability, the buff still appears in-game but it allows the ability to stack. So I renamed the Devotion Aura buff Shield Wall and made a custom Devotion Aura buff for the real Devotion Aura for paladin heroes; the Shield Wall aura ability is added to the unit by trigger. The only caveat I've noticed is that the ability, for whatever reason, stacks twice from a single origin, so you have to make the ability apply only half the desired armor.