I soldered some 10 ga wire last night onto a couple alligator clips for a 12volt contractor fuel pump in the bed of my pickup truck.
I used a gun type soldering gun and was able to easily have enough heat to get the allligator clips hot enough to flow the solder out. These clips go on 12V batterys, they are not small.

If the wire is corroaded or dirty it won't take solder.

I work on petroluem tanker trucks everyday, those hand held torches are scaring the crap out of me.