High frequency step up TRANSFORMER.....?

When you say powerful, I hope you mean high voltage not current.Tasars typically operate in the milliamp or microamp range because they are guaranteed to penetrate into the dermus (low resistance) parts of the skin and scramble nerve pulses. They have to potential to kill weak people and have done so.1 million volt tasars are available so why do you want to work in this area? Audio transformers are very frequency limited and risetime limited, both a stop to what you want to do. Flyback transformers are designed to provide a fairly high current, high voltage within a 17 KHZ range.Most of the time the 15-18 kilovoltage output limit is made more useful by running it through voltage doublers or triplers. If you have not been knocked on your butt by one of these or had a hole bored through your finger, I recommend it to try. It will knock some sense into or at least knock you off the workbench and prove some respect for high voltage and how bad it hurts.