As beautiful as Caster is, she is as equally deadly, given she will not allow herself to be taken by force or even approached on a friendly basis without causing havoc in the process. The best way to take her is to get her to lower her guard (often in ways related to Saber) and negate her magical powers in a reliable fashion. Since she's not a physical fighter, she's practically a sitting duck without her magic. Be sure to bind her in any preferred way, but don't let her hair get caught in any of the bindings. Use a gag that won't cover her weak spot: the back of her neck- another reason to be able to keep her hair loose.