I generally use the propane torch for anything over 8 gauge. Turn your heat way down and watch your solder flow. When the solder starts to flow freely remove the torch flame.
If you have never done torch soldering I would recommend a practice run or two! When you familiar with it its very easy.
When doing larger connections try to heat up the joint quickly and apply the solder fairly fast. Large wire conducts heat very well and can also wick solder up several inches into the plastic covering if the heat is put on too slow or held there to long.
You want to heat up the area your soldering, but as little of the rest of the wire as you can.