I think a mech would be just dandy for bear. If you wanna see what would be more realistic, shoot the spitfire through a jug of water before shooting the plywood. Shhoting the plywood alone can give false results as the blades could either
a: go through the hole...saving the blade
or
b: get stuck on the flat hard surface while opening. Unrealistic as most shoulder blades are INSIDE of the animal (under hide,fat, and some meat)
The jug of water will open the blades the way the hide/fat/meat would.
You could also try coconuts. the round shape doesn't catch the blades. I have found coconuts to be nearly as hard on blades as a steel drum (wait till I post my Muzzy/coconut pics).

As stated before...if you shoot in the vitals (NOT the shoulder) you shouldn't have any problems with the mech head.