(continuing discussion) One possible fix is to make upgraded fighters more expensive to train. Another possibility is to make it so that lesser fighter types are useless against advanced enemies, so spamming does not help. You could do that e.g. by adding the concept of armor (armor score is deducted from attack damage score), and setting the scores so that basic fighters are not able to penetrate armor of advanced enemies.
Nice game, but some balancing issues with the advanced fighter types. Currently there is no point in getting any Boxers etc. Just upgrade the basic fighter fully and increase the number of fighters you can have as much as you can afford. Then you just keep spamming basic fighters and upgrading the Dojo. Finally when you have fully upgraded the Dojo you can also fully upgrade the final fighter type (swordsman) and switch to that - although I'm not convinced it is really necessary or even cost-effective. Also, having some way to "win" would be nice. Once you have all the upgrades there is nothing to do except spam the Train button.
To win the game you need to upgrade all fighters and all dojo upgrades. I will see what I can do to balance fighters. It seems like everyone just spam basic fighter. I can make the second, third, fourth... fighter more expensive. After some point training basic fighter cost more than other fighers :)
 "Protect The Dojo"){kind=icon}
I updated the game and added concept of armor. It may be unbalanced since I didn't test it out much.