As per the Voduke science video, you should play on Casual, play in the last environment (The Underpass, The Ascent, Otil or later), and prioritize maxing out the most powerful skills (such as multi-hit skills).

In KRF in particular, The Underpass is basically perfect for hero grinding, since it has lots of weak enemies that don't deal a lot of damage, allowing your melee heroes to easily fight them for extended periods of time. There is also only one main chokepoint where all the enemies will come. The same is true of Otil in KRV.

KRO is more annoying because of enemies like Harassers and Sons of Mactans, so you can't just blindly send in your melee heroes to fight without dying. In particular, frail heroes like Razz and Rags are the most annoying. So I would try and have them fight weaker enemies like Sword Spiders as much as possible (like on the bottom path on the Ascent), and of course follow the other tips from above.

Kingdom Rush Science: How to Level Up Heroes Faster in Frontiers, Origins, and Vengeance!YouTube