How can I power-level my recruits?

In Defender's Quest: Valley of the Forgotten I have just finished Hero Mode++. I would like to complete all maps on Extreme without restarting from scratch but I believe the six standard heroes will not be enough for some of the harder maps. Is there a way I can efficiently power-level new recruits up to the level range 40-60 with minimal effort?

Solution 1:

The last level you were able to clear comfortably with your standard heroes on the highest difficulty should also be the easiest place to level generic recruits.
Just make the recruits, play the level as you normally would and rake in the experience points (depending on your choice of settings you may need to actually deploy your new recruits, but that shouldn't be much of a hurdle).

Also, congratulations on your Hero Mode++, I love the game but couldn't be bothered to push for it.