If you've been playing , you'll know one thing for certain: Bilewater is terrible. It sucks there. Enemies that explode when they die, darts that explode when they're thrown, bounce orbs that also explode—and worst of all, pools of maggots that will strip you of your ability to heal (after being exploded by said orbs).
It's also home to one of the most in Silksong's entire library. For the record, you can find a slightly-closer bench located in the screenshot below. It's down at the bottom of a shaft with a pool of worms at its base—just break the wall to your right instead of going left.
Even here, though, the runback sucks. Groal the Great, the boss at the top of Bilewater, can only be fought rummy 365 after several waves of arena combat against those same obnoxious bastards you've been trying to avoid just to get to the guy. I, however, have discovered an absolute cheese of a strategy: Embrace the maggots. Become one with the worms.
No, seriously.
You'll still need to put in a bit of work, unfortunately. Groal can still occasionally whomp you with this strat, so you'll want to go into your fight with him wielding as many health masks as you can. Otherwise, it works a treat.
First up, your loadout: I recommend the if you have it, but if you don't, just make sure you have some good ranged tools such as Curveclaw and Threefold Pin equipped, and use the Silk Spear.
For trinkets, I recommend the Pollip Pouch, which adds venom to your tool attacks and can be obtained by completing Rite of the Pollip wish from Greyroot in Shellwood. Bring plenty of shards, because this'll still likely take you a few tries.
Play the arena gauntlet conservatively, aiming to take as little damage as possible. Do not use your tools—slow and steady wins the race here. Your objective is to get to Groal with as much health as you possibly can—then surrender. Surrender to the maggots. They may prevent you from healing, but they cannot hurt you.
If you simply take a swim in the bottom left or right corner of the arena, a good 3/4ths of Groal's attacks simply cannot reach your itty-bitty hitbox. They'll just sail right on over.
The only thing you need to look out for are his breaching and entering attacks—he'll sometimes dive right into your face—or his firebombs, which can occasionally land close enough to be a problem. If they hit the water, they'll simply vanish from existence, but if they detonate just above it you might need to move.
Here's some video proof of me beating him even after messing up and getting swallowed (at the very least, you can smack his insides to hurt him when this rummy 51 happens) and then doing a little celebratory dance. That bit is very important.
If you're sensible, you might want to visit the Purified Ducts via The Memorium and simply come back to Bilewater later. In which case, you can also snag the Wreath of Purity from there before attempting to push Groal's stupid face in with your knives—this handy charm prevents maggots from nibbling at you. As you can tell from the footage above, I did not do this, because I am not a sensible person. In fact, rummy best I am covered in worms.
The Wreath gives you a solid buffer, but it'll break upon contact with too many maggots, but if you follow the strat above, it might give you the few extra health masks you need to make a difference. Good luck.