I had a similar situation with Liriope spicata. Sprayed it with RU and it just chuckled at me. So I tried beating it up, roughing it up heavily with a leaf rake first, causing the surface of the leaves to be scratched and scraped, then hit it again with RU. That time it had an effect, apparently being able to penetrate better. Still had to repeat several times but eventually I got it all. You might try that technique and see if it works on mondo.