King fish rig up

The last time I went out I was fishing by a rig roughly 20 miles out. The water looked great! Pretty and blue and calm. I tied off to the rig and started bottom fishing. Caught some great snappers but had to let them go due to season being closed. I had on rod out the back free lining a dead whole cigar minor. Caught a nice king 52 incher. Hooked a huge cobia but lost him at the boat. Here is my question... I had lost 3 hooks due to something with teeth. Could be shark or kings. So what kind of rig should I use while free lining?? Should I have been using custom steel leader? If I do, will cobia and mahi still take the bait?? Should I use a stinger? I'm not really targeting kings, just whatever takes the bait. When I did gaff the 52 inch king, as soon as he hit the deck my line was cut and chewed up bad.. Line looked the same when I lost the other 3 fish. I was using 300 lb mono 4 ft leader.. Any recommendations??? Thanks everyone! Good luck fishing!