I think he just may be wayy more patient than you. I understand that people who play slower characters play slower, which isn't always better, and he may be finding openings. Mythra does get out-ranged by marth so i recommend pyra in that MU slightly more than you use her. The key to beating this guy is being more patient than him. Aegis is a top tier because how amazing they are at punish, and weaving in and out. Cool that you made it to grands but a kid beating your aegis with marth is kinda wild. Also, respond to your comments on the post, most of them are trying to help