Question about individual training and the fastest way to improve attributes

[y0lo]

I have a player with low stamina and really it is his only weakness. He is age 20 with room for growth.

I'm trying to build this stamina up as fast as possible and I do not care so much about other attributes.

In this case, what is the fastest way to build stamina out of these two options

1) Put player on a position/role/duty training that includes stamina and as little other attributes as possible + individual training for Endurance with intensity level.

2) Individual training for Endurance only

Let us say, we keep the same intensity level (Double Intensity)

This example discounts the impact of Team Training where you could prioritize fitness for example.

So a wider question, applicable to other attributes would be - does training ONLY "Additional Focus" (e.g. Endurance, Final Third) give you faster improvements of the attributes highlighted than combining with a "Position/Duty/Role" training.