Alright so my Marth is purely support and I gave him these skills:
A - spd/def bond from ethlyn, he’s always close to others for buffs so bonds work great. I like this one for better survivability, reduces damage + avoids doubles. Atk/spd would be better for offensive, but I prefer spd/def to outlast his opponent as he heals 20 every other turn and can take a hit while his other team mates finish them off. Maybe sacrifice a Rutger for atk/def instead if your low on feathers/grails?
B - renewal, fulfills healer role.
C and S - drive atk + spd. I pair marth with NY!Azura with drive def + res, It’s a great combination if you have her thanks to her weapon. Together that’s +5/+5/+7/+7 to an ally.
Finally his special is blue flame - goes well with the bond skill, if you can get owain that’s an atk/def bond too. Otherwise stick with Moonbow as he shouldn’t be seeing much combat.