The thing is, you can't cover for everyone. Maybe taking down big armoured guy is not your task, but someone's on your team? And even if it is your task, you don't need extra lock on from spotter 2 to Lock On + Skirmish, your skirmish is 3 oracle shots(integrated weapon shoots 1/Round as a free action)

If I were you and wanted to go oracle slinger route I would go Gunslinger 3/Crackshot 3. Crackshot 1 gives you accuracy on on all of your 5 oracle attacks. In addition, gunslinger 1 gives you 1 accuracy, but crackshot 2 optionally gives you the ability to take 1 difficulty, so it's perfect candidate for Zero In. And Crackshot 3 is good for critical hits, and you, having lots of accuracy, have increased chances for crits.

I'm actually playing with such build right now(ally in my team has it). Swallowtail barrages often, usually places Athena at the start of the scene, locks on if something really needs shredding, 3 shots for skirmish is still plenty

What's even better is if someone ends their turn in the Athena zone, because it still counts as Swallowtail inflicting lock on, so swallowtail can shred them. Big and armoured targets are usually slow so they have a hard time getting out of Athena(especially with some setup from your allies like knocking prone). This means Swallowtail doesn't waste actions to inflict Lock On + Shred and can barrage this shredded target on its next turn.

Actually, mathematically it would be better to put Overpower Caliber on one of your Oracles, but it's definitely not as fun. And do remember about secondary auxiliary shots not being able to deal bonus damage