>everyone here is getting downvotes

Get a piece of equipment that lets you start with a beam sword. You can't pick up other items as long as you're holding it, it can be knocked out of your hand, and you lose it if you die, but hey, it's better than nothing.

In the 3DS version, you can get a Beam Sword item just from doing a challenge. Get a 40-hit combo in Training Mode, I think it was? Easy enough to do if you can set up a chain reaction of Smart Bombs. In the Wii U version though, there is no such challenge, unfortunately. You best bet is to do Crazy Orders. Picking tickets marked with the Fire Emblem series logo might yield a Beam Sword Sword (redundant name, much?) if you're lucky.