How can you most effectively kill creatures to gain experience? For example, you've gathered 125 medusas. Here are some options:
1. Send them into the thick of battle.
2. Use Meteor Shower.

125 medusas – against demons? =) Not bad. That's quite a powerful attack force! But you've already answered your own question. Do whatever is most convenient for you. It's good when you can use them to deal damage to demons – you kill two birds with one stone. For example, if you're fighting sultans and ifrits (a tough challenge for me), you can't avoid losses, so we try to feed them with expendable units. Just make sure you don't lose any valuable creatures in the process.