I have been playing through without continues, keeping track of my high score, and restarting when my game is over. I can get to the first part of the last stage, but die miserably from those large bat things. Without trying too hard, my scores reach around 150-160k.
I always knew that killing several enemies with one throw of a weapon netted bonus points. I would do it when I could with the boomerang as I went through the stages, but I never spent much time with it.
Today, I was working my way through the stage, when I realized that the "fire bomb" (I had to look that one up; I still call it holy water) also gave the bonus points. There is a point near some stairs at the end of a section where the zombies just keep coming if you stand around, so I milked some points there, then moved on.
Then I found a much better place to do this. The very beginning of the final section before the boss, you can stand on the bottom left and throw fire at the oncoming groups of three to get 3000 points per group. This area is slightly better than the previous area, because the zombies return immediately after death. I'd estimate you get about 1000 points per second. I stood around there as long as I could before time started getting low. I ended the stage with almost 100k, and that was without rushing. I can imagine breaking 200k if you are determined.
There may be other areas just as good or better than this. I'm not sure how exploitable this is with intentional deaths, as you start with zero hearts. I suppose if you had a seed of 10-20 hearts, you could generate enough from monster deaths to keep going until time runs out.
I will keep an eye out for an area that has the firebomb, a decent amount of hearts, and infinite enemies. You could probably get unlimited lives in this manner.