A follow-up that has probably been covered but would love to make sure I'm doing this right:
I Focus my TIE Fighter to have it strike.
LS has no units at the current battle so he passes.
I play Talon Roll to remove the focus token from my TIE.
I Focus my TIE Fighter to have it strike again in the same battle.
Is this all legal?
Yes. That's pretty much the purpose of Tallon Roll. And as there's an action window after each unit strikes, you'd be able to play it during that engagement. And since the engagement would still be ongoing, the TIE would have to strike again.
Of course, you could wait until that engagement ends, then use the Tallon Roll so the TIE could engage a second objective.