SAMURAI WARRIORS 5 (EP)

I used No and got this achievement very easily. I think I got a combo over 25,000. Just use her X,X,Y combo constantly to rain down arrows in an area and keep spamming it as you slowly move forward. I got the achievement on the 4th level in part 1 of Nobunaga's story in free mode from memory. No was around level 15 or so and I had unlocked all of her skill tree that I could too.

The trick with this is that if you have two characters you can swap between them and the combo won't go down. So if you're in an area where there aren't many enemies, just swap to the other character. The AI walks to enemies by default (unless you set them to "stay".)

Here's a detailed video: