The main thing with large guage joints is heat transfer tin both surfaces with a fair bit of solder then leave the iron for a few minutes to heat up then tin the tip with a fair bit of solder this will ensure you get a good transfer of heat. You should never have to apply any pressure to any joints with the iron. Some irons such as my weller have different number tips for different temperatures weller and antex are about the only 2 irons I have got on with. I have tried a few solder guns and found them to be very poor the tips never heat in the same place twice I found my old 12watt weller worked better. Hope this helps.
