I'd definitely try a flamethrower or other quick burning methods if it's a battle. But if he/she is restrained, then I would just have to try a knife with a vibrating blade. They hate high frequencies of sound, or in other words, air that is rippling.

If I introduce a nearly supersonic steel blade to them, what will happen? I theorize that the symbiote will try to get away, leaving the host underneath vulnerable to it. Ultrasound torture is also something I'd want to try, or at least to see what is under neath the symbiote skin.

But yeah, flames. Or maybe cook them in a giant pressure cooker, and catastrophically depressurize it. Should make for a fine spectacle.