Apart from the insane detail (you must have a pair of eagle-eyes...) it's the blending that's fantastic and causes me the most frustration so....how do you do that...
For acrylic painting of flesh (where you want perfect blending most of the time) here is my technique for figures.
Apply a uniform basecoat with the shade you want to be your middle tone.
Mix 3 darks and 3 lights.
Apply 1st dark then 2nd dark in a very sketchy way without caring at all about smooth transitions
Do the same thing with the 2 lights
Now, this is where the smooting will happen.
Use a little thinner version of the basecoat and reapply it at the transition between original basecoat and 1st dark and 1st light.
Mix a tone between 1st dark and 2nd dark and apply it between these two zones
Mix a tone between 1st light and 2nd light and apply it between these two zones
apply ultimate 3rd dark and 3rd light
So in my technique I do not use any ultrathinning of paints where you apply dozens of coats..it takes way too much time
For the cape (red part) technique used was a little bit different :
Uniform basecoat with the airbrush
One lighter coat with airbrush
One darker coat with airbrush
Freehand added with the brush using even lighter and darker tones
So in the end, what you see in the cape is 80% brushwork and 20% airbrushwork (mainly done for rough sketching)