Ciphers use 'Focus' for their spells. They start any battle with a bare minimum but as they attack with weapons it creates more focus. A great weapon for this is the blunderbus because it fires multiple slugs that all count towards gaining focus. Each spell has a focus requirement, that is why you can't cast some spells. Do not forget that some spells are also once per rest or once per battle, etc.

Don't let any of this put you off ciphers, though, because they are mega powerful, particularly as crowd controllers.