(Spliced) Beating Rusted Moss with no upgrades [Sub 6] 

Spliced upgradeless any% run, using Maya (no grapple hook !!!). The HP upgrade at the start isn't needed and was a mistake (oops). I'm not the first person to do this, but I can guarantee I'm the worst person to do this. Extending my time to 6+ hours is an exercise left to the viewer.
Most of the platforming is straightforward, but since I'm new to Maya it looks suspect. It's basically a modifed any% route, skipping Bonnie and both weapons since we effectively start with grenade/shotgun/rocket. Ledge drifting is used in two spots (you could probably do the second normally, but I'm bad), you want to light attack right as you're grabbing the ledge. Some enemy AI gets disabled if you're far enough away from them, which I abuse for the worm room and the post-credits scene.
Seer with no upgrades is an experience. Grapple is a huge crutch for dodging her attacks, especially in phases 1 and 5. Maya's lack of consistent healing also means you can realistically only get hit once, so getting hit early is an automatic reset. If you're good, you can hit her when she's in the wall to make the ghosts get stick in the walls (they don't despawn) for free HP later (used around 16:10).
Seer 1 is the bane of my existence, her gray attack requires moving constantly while the star attack wants you to stay still, so if you get unlucky she can spawn right where you're moving to for free damage. Most of my resets are to early damage on that attack, or getting got by bad star RNG (dodging properly is for nerds). Seer 2 is easy, so much time to dodge and many windows for easy hits.
Seer 3 is also easy, with lots of attack windows, but I've lost a bunch to getting cocky going for HP. Seer 4 is a menace thanks to the massive gray flower attack, but is similar to Seer 3 for damage. Every other attack is easy. Seer 5 is completely scripted. Jumping into the air for the gray flowers and pink blast makes them easier to dodge. I wiped twice to grenading myself with less than 50% remaining, so don't do that.
Overall, I don't think this is very useful from any kind of practical standpoint. It is somewhat interesting how little the grapple hook game requires the grapple hook for standard content, but anyone with an hour using the rocket/shotgun will know that. For any% viability, getting the grapple hook and HP Overload will save you time, especially since you can probably skip both ledge drifts, and makes the boss much easier.
This is a callout post to someone cracked at the game to get upgradeless mossy platinum.