- try to play below Max Power for as long as you can (because Max Power raises rank). For me, it was until stage 3 before midboss Meiling iirc.
- try to learn some spellcards and nonspellcards and time them out if you can (some of them aren't as much difficult to timeout, but even so, it'll pay out later). For example, you can timeout Cirno's laser+bullet stream nonspells, Rumia's spells and nonspells, Meiling's doll streams sc, Meiling's rainbow bullet spiral sc, Remilia's big red bullets sc (one of her first scs or nonspells, don't remember now, where she either fires aimed bullets or starts shooting bullets at fixed directions/angles; you can option select here by standing at the farthest upper corner of the screen and seeing if she has fired the aimed big bullets or the fixed big ones), Remilia's spiraling knives.
- try to learn some of the stages' safer zones (upper corners near the end of stage 3, where a lot of fairies shoot downwards, upper half of the screen above the maid fairies on stage 5 which go from one side of the screen to another shooting downwards, middle of the screen on stage 6 and in some places of stage 5 where fairies come from both sides spreading rings of bullets with aimed (micro-dodgeable) bullets
- try to plan your bombs (for example, on patterns with high clipdeath potential such as Meiling's spiraling kunai, or somewhere in stage 4 where there's kunai and lasers and you're supposed to micro stream the kunai)
- learn stage 4's enemy placements (there are some enemies on it which shoot away from you, there are some enemies which require you to know where they'll come from -- bad memory from me, can't help you here)
- make sure you've brought enough lives and bombs to stage 5, don't think about playing for score (especially since doing so requires you to further memorize opportunities for grazing and spending bombs for clearing bullets and using their invulnerability time for point-blank grazing)
- Marisa B is an option if you like the idea of dealing high damage on bosses with bombs and can align your laser with them (this should make a difference on the latter bosses)
- like Jaimers said, try to get used to the bullets' hitboxes, they can be confusing and limiting if overestimated, and deadly when underestimated. A frustrating step, I know, but really needed if you want to have better chances to 1cc this game.
edit: this
replay was done in 3 days iirc, during the SotW's Varth week. I really wanted to play Varth but had engaged in a challenge where I had to 1cc EoSD Lunatic before my friend reaches DDP's second loop (in other words, exchanged challenges). Played Varth for the remainder of the week, had LOTS of fun with it since it was the first time I got past stage 17, ever since knowing this game on 2009's STG Tournament on the shmups.system11.org forums (on which I've participated for the first time too, so Varth is special for me, especially because it was the game for STGT '09's first week), and reached a score which is imho really more difficult to attain, than reaching DDP's second loop (a bit over 5,000,000 points, on stage 25 -- everyone should have a chance to fight the boss at this stage, under high rank and your last life, which I had to maintain shortly after derping the extra life I got at 4,500,000 points. Lol @ my brief sense of relief. 1.50 Lagless 99p input file
here.)