Didn't grind a single skill. Just grinded levels and money dlcs... then go into the rankings for Castles and clear without handicaps and buy skills. Easiest for the main royalties and most children, rarer for the underused characters like Crimson, Asama etc. Pretty much got all the skills I'd ever need. Solar, Luna, Comet, Aegis, Pavise, Hoshido, Nohr, Dragon Curse, Growth... although you cannot BUy dlc skills and still need to earn them, although Aether is easy as hell to farm.