Mages can tend to have good staying power if they are a bit blessed with Defense and Resistance, luckily the Hoshidan Tomes can give good defensive boosts, like the Horse Spirit, i would recommend you go some sort of Mage, then maybe grab Vantage, try grabbing Tomefaire, a good offensive skill like Dragon Fang or Luna, a defensive skill like Pavise and the last skill can be whatever you want, if the online still works for Fates, look around in other peoples castles and see what kind of skills they have on their avatar, if you have a good sum of gold or grinded the gold dlc, you can always buy yourself a nice skill build