You could, but it would take a while. There's a explore mode where you just go around doing battles. But you get better xp by doing actual quests. There's side quests too that are usualy lower level requirement than the mains. So you can kind of use those to grind. But you don't realy have to be worried about being under levelled imo.
Another thing is with the online you can "dispatch" troops while you're not playing and they will gain xp doing that. but you don't gain a whole lot doing that. It's fun though. Every night when I'm done I send out my units like that