...Striking it dead center in the breastplate, and leaving a dent. The suit of armor actually staggers slightly.

That's all the opening Felix needs. He levels his sword at the Dullahan, concentrating. Two spectral blades fly out of nowhere, slamming into it at oblique angles. A considerably larger sword of light forms around the blade Felix is holding, and he charges at the now-battered foe. As the outer sword touches the Dullahan it shatters, its momentum sending dozens of magical shards into and through the armor.

Said armor slowly collapses forward, taking no action of its own, until it comes to rest on one knee. It doesn't move.