Dedenne is a powerful thunder type Pokemon and while he doesn’t seem to be on Pikachu’s level; he’s still a dangerous opponent. Marth is a powerful swordfighter, but he lacks the feats that the greats like Roy posses. Speed is one of Dedenne’s big weapons and Marth just won’t be able to match that. Marth will go down against the laser blasts and eventually be taken down for the count. Dedenne wins.

Update 2/7/2019 Thanks to Fire Emblem Warriors Marth is strong and fast enough to win this round. Marth wins.