The other Achievement guides didn't work for me, so I switched to
Gaira, the large character on the top-right corner of the character selection screen who uses beads as a weapon. I found him much easier to get perfects with than Shizumaru.
Select Gaira, then Slash, and Beginner. When the fight begins, just jump at the enemy and
use the Y button jumping attack to deal massive damage. It should only take 4-5 jumping hits to defeat the enemy! The goal is to get two perfects, preferably during the first fight - they award 50,000 points per round. You might still have to retry a few times, but it shouldn't take too many attempts to get those perfects - I got it after two tries! After your perfect first round, just fight the remaining battles, using the same strategy to do your best, and you should hit 200,000 points before long.