I second that...hit them in the head! Right through the gill plate is a good place....but the best place, and also a large target on the swords, is through the eyes! I usually try to get bigger swords that I think might have some energy left in them right through the eyes...it is a very strong pulling point. On the shoulder you will ruin meat, I hate when that happens! And if it is a nicer fish and it starts thrashing around, your gaff can rip out because of how soft their bodies are. Also opt for a 4" hook and a 6-8' handle...
Justin
Canyon Runner Sportfishing