Due to personal preference, I mostly play as Dry No Legs. What's more important is your costume and sub weapon.

If you play as Dry No Legs, you want to equip "Prickly Leaf". It increases your invincibility time after using a bomb (you'll equip the cheat "Auto Bomb" which makes this all the more useful). It also increases the invincibility time at the beginning of a level, which allows you to speed kill enemies of the first wave.
Most characters have a costume with such an effect. Use them, they're awesome.

The great thing about this weapon isn't just its spread. The bullets themselves are quite slow. In order to take down enemies of a new wave much quicker, you should spam this weapon until the sub weapon meter is completely empty before collecting the diamond of the previous wave, which will fill the meter to 100% again. Then, you can just go on spamming it again!

Obviously, this build relies heavily on special items dropped by enemies at random chance.
Therefore, the build is less effective during boss fights, unless you've got incredibly good RNG (which is how I managed to get my 1 star in the final battle).
The difficulty of levels that have fewer but stronger enemies will also be less reduced.
I equipped 1 extra bomb in this build for extra safety. But if you're confident and better at dodging or prefer to rely much more on RNG than bombs, you can unequip extra bombs and "Foes Drop Bombs" to boost your scoring.

Yes, we're going full bullet speed here! You should also equip "One Heart". If you've got good RNG, you won't be losing a heart anyway.

Since Bullet Speed will also cause enemies to appear quicker, these handicaps will work in your favor if you have a shield/sword or post-bomb invincibility.
You might even want to consider equipping "No Bullet Clearing" on some stages, unless the projectiles are just going to stay there forever.

Since this build is basically a workaround for dodging bullets most of the time, it'll help you get a good ranking on the leaderboard, especially on the later levels that are harder.
Boom! #48 on Level 10-4 on my 3rd or 4th attempt.
On most of the harder stages I'm within the top 100 or top 50, even though I didn't spend time learning patterns at all.