It's important to note that words ending in "-er" aren't always automatically given feminine forms. This is because many "-er" words are nouns that describe a role or occupation, regardless of gender.
Here are some ways to address the need for a feminine form:
* Use a neutral term: Instead of "caster," you could use a neutral term like "person who casts" or "casting professional."
* Use "woman" or "female" as modifiers: You could say "woman caster" or "female caster" if clarity is needed.
* Contextualize: The context of your sentence might make it clear who you are referring to without needing a specific feminine form.
Ultimately, the best approach depends on the specific situation and your desired level of formality.