In places like oluns and gyfin lower it's hard to keep track on the elite mobs healthbar. Especially in oluns because of how tall the enemy is. This makes it hard to sometimes keep track of the health and when to stop DPS before the arm phase. It would be nice to lock onto their healthbars to avoid killing them too fast.

This would be incredibly helpful to have for the reasons listed by Tonakai. The main enemies at spots like Gyfin Lower and Olun's are tall enough that their over-the-head health bars are off screen. When you're being swarmed with adds at the same time, you can't reliably depend on the on screen health bar since it flashes between all enemies you hit. If you could focus it on specific enemies, it would be a huge quality of life feature to have.