-Def +Spd Marth

Definitely not. Marth has pretty good defense, so a def bane is really something you do not want unless you're going for some "out of this world" build that sacrifices all defense for offense or something.

-HP or -Res are the better banes for him.

Now, unless you're planning on merging Marth's for a 5* +10, or whatever merge number, then go ahead and 5* him, but if not, then wait for a better one.

HP is his best bane, don't mess with his RES since he will be slaying dragons

If you're running his usual Fury double Renewal set it won't be TOO TOO bad, just try to pick your battles more and probably pack some def buffs (or Close Defense seal) I run a -HP Marth, which is also a superbane, and all he really needs to do is just be a little more careful