Scarlet Gensokyo is still the longest seeming spellcard ever. Unless you pull off Nintendonuts cheese trick of bombing scarlet meister at 1% or whatever it is, I haven't tried that myself.
The trick is just to shotgun Remilia directly after Meister ends. There's a period of about a half-second where she doesn't have spellcard armor, so you can usually take out about 1/4 of her health that way. Oh and make sure you aren't using a bomb when Meister ends, or Remilia will be in bat form and it won't work.
Also, be sure to collect the big powerup right after she shoots the first wave to knock it out, and whenever you bomb or die, stand on top of her during invincibility to knock out the big wave that she usually shoots.