I think we should be able to fight polymorphs before getting them.The gobbler would be easy and so would be the mander.The cat bandit would be really hard but if you get some good storm resist you should live through its big hits.
I mean like before you get the ploymorph spell you can have an option to try fighting it to see if it is worth a training point.At the end of the battle you have a choice to get it or not.
