RE: Spine check please?!
I love mechs, but IME you need some horsepower/overkill for them to be scary effective- 60 pounds of KE at least, and 65-70 is better. Light arrows, low poundage and mechs can (and often will) yield some poor poor results- def stick w/ broadside shots and keep em close. 20 yards- if you are set on mechs and light arrows.
My reccomendation with your poundage and lighter arrows would be to stick to a good cut on contact fixed blade (Phantom, NAP Razorback, Steel Force, etc etc), or smaller diameter Muzzy, Wasp, Slick Trick, etc.