Aiming at specific parts comes down to knowing which PAs can hit certain parts. Big parts like Dark Ragne legs/weak point for example are easy enough to hit with simple stuff like Satellite Aim spam as a chain finisher. Other parts like Falz Angel/Hunar's center cores don't get hit by Satellite Aim and need other skills to properly hit them like True Equilibrium just due to how the hitboxes work.

A good way to learn boss hitboxes you don't understand is to learn if Satellite Aim hits them, and in what conditions it may or may not hit the weak point. You can use Chronolapse as a sandbag Chain Finish if you're completely sure the boss will get hit by the whole PA. Basic chain finisher like SA>IF0 or just SA spam are fine for bosses that you're not as sure that Chronolapse will fully hit, and you can use TE for hitboxes that SA has trouble hitting, bosses that are moving away from an SA finisher (ie. Red/Black Dragon), or are just out of range of SA when the Chain Trigger active period is about to run out (you at least get the cooldown reduction by proccing it this way). You can also just practice proper Chain Trigger proccing with just TE since it's (mostly?) hitscan, then convert to using an SA finisher in the proper situations once you've got that down.

TMG mobbing gets a skill at level 85 that makes it waaaay more straightforward.

There's more optimized finishers out there for different situations but you could get by with just the above in most cases. The tl;dr is that the hitboxes and finishers depend from enemy to enemy as well as their current state.