The head is symmetrical right? Delete half the model along the axis of symmmetry, this will instantly half the amount of polys you’re working with. Play about with default cylindrical and spherical UVW mapping (pay particular attention to where you place the green "seam" on the gizmo – it can make or break your unwrap). When you are reasonably happy, then go into the UVW Unwrap editor and relax UV clusters so that no polys are overlapping. When you have a UV layout you feel you can worth in Photoshop/similar, mirror the half head model, attach the new mirrored model to the original and flip the UV coordinates horizontally, then you can weld the line of symmetry in the UVs in the UVW unwrap editor and paint away.
You should end up with something like this, but try and keep the seams vertical on the extreme right and left sides: