yeah, I kept looping at the boss fights too at Round 6. So really it's just a matter of patience... I can't imagine how long it would take to roll the score, but when I was playing I gained a free life every time I looped, so as long as you always get to RongWang The Puzzle Muster and let him kill you, you will always regain the life you lost letting him kill you, you CAN'T get Game Over really.
Here's a score without Round 6 loop abuse. 379100 points. I guess the key to getting a bigger score is to take less damage and clear stages faster while at the same time still murdering everything you see.