All of my speakers are also connected by 12 AWG. Do bear in mind that stranded 12 gauge is considerably larger than solid core. I have used a simple diagonal cutter for decades to work with these wires:
Milwaukee 7 in. Diagonal Cutting Pliers 48-22-6107 - The Home Depot
For smaller wires working with line level stuff I have purchased some of those fixed gauge strippers/cutters. They do a great job on the smaller wires, and better than I could do by hand.
You say the outer shell insulation is pretty tough... I have to ask why. What brand did you buy? With low ohm speaker wire the outside insulation has little to do with sound quality. Good quality speaker wire should be flexible & easy to strip.