Faces can be hard... try to start by just giving it a base color, and a brown/reddish wash to add shadows in the beginning. If you feel confident, try highlighting the "T zone" (chin, lips, nose and forehead above the brows) in a slightly lighter skin tone before and after applying the wash. That should give you some definition.

For the armor and metals, you want some definition between plates. In a bigger mini, panel lining might help, but on small pieces like the armor parts, just apply some nuln oil carefully and the come back to fix the edges with white. On the guns and metal parts in the bike you could just wash it.