So, has anyone tried scoring in this game yet? As far as I can tell, it's all about the bullet canceling. The clear bonus appears to be a static 120m, and spell bonuses are negligible, so it's best to spend as many resources as possible.
In particular, the final card is worth an absolutely ridiculous amount of points if you bombspam the quadruple Master Spark phase. From testing it on Normal stage practice with Sanae/Kanako-A's Kanako bomb (which hits the entire screen for a decent duration), it was worth about 90 billion. (Remember to keep shooting so your multiplier stays at 999. Mitama has so much health that you don't need to be concerned about defeating her early.)
Of course, in a full run, you won't have as many resources as in stage practice. So the rest of the game seems to be about conserving as many resources as possible to use on the final card. However, with the Kanako bomb I've been testing with (other characters may have longer or shorter bombs), 8/0 or so is enough to last the entire timer, so any additional bombs should be used earlier.
Timing bullet cancels should also be relevant, though it's more difficult than usual, since if you stop shooting, the multiplier drops below 999 making the bullet cancel pointless. One strategy I've found is to unfocus to reduce your damage while stalling for the right time, though you have to be looking at the health bar well in advance for that.