I don't quite understand why you say this. I'm familiar with inheritance vs composition, and IS-A vs HAS-A. It seems to me that this is clearly an IS-A situation.
Wizard IS-A Baddie
Knight IS-A Baddie
Slimer IS-A Baddie
The problem is that you use IS-A in the way we normally use the term in natural, spoken language. However in OOP the term has very specific meaning. I personally blame bad teachers for this misunderstanding, as inheritance is often taught with contrived real world examples that never actually occur in real software projects. (Plant-Flower-Tree-Bush for example)
Make yourself familiar with the Liskov substitution principle. This should be the benchmark to decide wether you should use inheritance or composition. As a general rule of thumb: When in doubt, use composition.