Holy Water
But other than that the game is mostly doable by just knowing when to jump and when to attack. It's a pretty rhythmic game. Frank and Death are hard because of Igor/scythes. Frankie and Igor are somewhat manageable with just the whip, only because Igor freezes when you hit him for a couple seconds. If you're good with the i-frame timing, you can lock him while you mash frankie away.
Death's scythes are a different story, they're random and you really can't help if two of them spawned just over you. Your best bet is getting there in full health (doable but hard), and then wish for luck with the scythes. I never tried a whip-only run, but the first time I beat Death, I used triple cross, I refused to use Holy Water thinking it was like cheating. But now if I want to blast through the game I just lock Death with holy water and go to the next level.
Levels 1 to 3 are really easy once you know them.