This really depends on how much we want to balance availability and power. Honedge isn't particularly rare, Aegislash is pretty amazing.
Its stance change ability allows it to vary between Shield Form and Blade Form. In Shield Form it has 150 base defences and (I think) 60 base attacks, and starts the battle in this form. It's very tanky and can use status moves (like Swords Dance) without changing form. Upon using an attack, it will first switch into Blade Form, swapping Attack/Defence and Special Attack/Special Defence.
I think it fits in well at Complex/Demanding/Merciless. From my experience its definitely one of the top tier Gen VI Pokemon that should be that high.