It would be the best route, but time consuming. Look at each player's attributes. Does he have a stand out weakness for his position (eg an AMC with creativity 3)? If so, you'll probably not go wrong giving him intensive training in the relevant section. In my example, it would be attacking training. Alternatively, you could just train him in areas that would be good for his position.
I don't generally train indiviuals unless I have a player who really needs to work on a specific area. I have a reall gifted 16 year old, but his physical attributes are really low, so I've got him on high strength and aerobic training. He gets injured a fair bit, but his physical attributes are now at an acceptable level. It can be very beneficial!
I don't train individuals, but I got like a dozen or so different trainingscheduals for my players. I look at their attributes and decide what they need the most and put them on a trainingschedual that reflects it. Sofar in 4.5 seasons I have had a total of 2 players performing bad in training.