When I play casters I will not touch a mob <10-15% as there is a large chance it'll be dead by the time I cast, just move on to the next (if multiple mobs. This also applies for all jobs if you have 2-3 mobs and one is 10% with DOTs, ignore it and move on and let DOTs finish it up)

For PS4 you can use the D-pad to cycle targets.