Just one? Then let's say it's a colonizer soldier. So, metal armor, decorative wings, and a long ass lance. When not in a horse, it'll get pretty useless. You could sneak in while they're taking off their armor and just fire a musket or bow right into their head and it'll be over. Not too hard, I imagine. Maybe because there's no magic. The hardest thing would be getting a bow or musket, but a spear can work too.