When the subject line reads "Hmmmm...", you know you're not likely to get a definitive answer. But I'll speculate based on years of workbench experience with a slobbering stick...er, soldering iron.

I wouldn't use a torch for electrical work. Too sloppy, too likely to burn the insulation off the wires for several inches, too likely to leave contaminants behind that will cause the joint to corrode. (Similarly, that "rosen" in the core of most solder, or that you brush on, will cause corrosion too; always use solvent to remove the crust after it cools.)

#8 is a pretty thick gauge, but I think it's right on the edge of what a 100W gun can handle. The trick is the tip: The larger it is, the more heat it conducts to the wire. The default tip is usually just a loop of copper which radiates most of the heat into the air and concentrates only a little on the contact spot; see if the kit comes with larger tips, especially one with a chisel tip.

Time is also a factor. The longer you leave the iron on and in contact, the more heat will build up.

You want to solder the wire right into the contact, but maybe that's aiming too high. Wire too big? Try soldering just the wire itself, to flow solder in among the strands, to make it solid. Then try filing it down until it fits into the connector, and screw it in as you're supposed to. You'll have few broken strands on the outside, but compensated for by filling in between the strands with solder.

None of that might work, of course. Just speculating. Good luck, bowser.