For me, Finn is quite easy to train. He's pretty durable, only because he dodges a lot once his HP is near critical. Slap him with an Elite Ring if you can afford it - I manage to get him to Duke Knight before chapter 3 (he leaves my party in gen 1 at around level 25, making him pretty solid in a certain chapter in gen 2).
And yes, about the Hero Lance, if you do pair him (I do suggest pairing him with Lachesis, because I think prayer is pretty good skill for her kids...?), make sure to sell it before he leaves. If you missed selling it, I think you can still get the lance in chapter 8.
I do want to argue about AideenJamka though. This pairing is still pretty good, weapon wise (and with JamkaBrigidd in mind). JamkaBrigidd will result with Faval getting both Ichival and Hero Bow, which is a bit overkill. Lester definitely can do great in early chapters with Killer Bow and Hero Bow in tow, and his accuracy is actually decent, if not good.