You shouldn't ever be using the Legends 1 lock-on at all. It's useless.
For that fight, just up the range on your buster. The normal buster has a ton of auto-aim, so with enough range you can just run and jump around while looking somewhere vaguely close to the enemy and holding down the fire button.
You could also try the Active Buster if you have it. You can't move while firing, which is a huge disadvantage, but if you upgrade the Special stat then the missiles will home in even more strongly than the normal buster does.
Also, whenever you're about to be hit, just roll. The dodge roll has crazy long invincibility.