I can totally see that shading/highlighting them by hand would be a bitch. Defo not something id want to try myself. An airbrush would suit them really well (ab's do well with smooth gradients over larger surfaces, eg, well, myrmidons, but that's a bit academic anyway, as it would appear you don't have one)
What you could do is take the chance to improve your blending skills. Big surfaces like the shoulders are easier to work with in that regard. If (and only if) you can be bothered, take some pics of the 'don being lit from above. Move the camera around the mini to take shots of the side/back (so make sure the mini is in the middle of the room, or similar). Print out these pics and use them for reference as to where dark/light should go. To shade, mix some coal black with some water (about 50/50) in a palette. Also get some of your base coat in a palette, also 50/50 with water. Paint the shadow color into the shadows on one shoulder only. While its wet, paint some of the base coat beside it (or above it - put it where the line should transition from dark to mid). While both are still wet, use a clean, damp brush to kinda swizzle the two colours together at the transition line. This reads as fiddlier than it really is. You will not get a smooth blend, but you should see the start of one. That's ok. Now, water down your two colors and use glazes/general fudging to smooth out the transition. On small areas this technique is a pita, but on big areas is works pretty well. Gie it a wee shotty - you might be surprised
They'll be green and obscene when you get your kitties ready to fight. 
