Ultimatecub wrote:
4 Sungrass, 2 Dreamweed, 3 Sorrowmoss, 3 Firebloom

Oh, I was just about to say, that with 4 Sungrass seeds, you could take either approach first, since each seed basically acts as 1 Healing potion, but I see you've already went ahead with fighting the fists first; and, as I've predicted you've already burned through half your Healing potion supply.

Also, for future reference, you could've tossed Firebloom & Dreamweed to the R-Fist, while using the Sorrowmoss & P-Gas potion on the B-Fist.

...1 unidentified which is either Mirror Image, Psionic Blast or Challenge...
...but if they're right next to the eyeball, or the B-Fist starts attacking as soon as I step off the stairs, it could probably be tough.

Well, here's the thing; you could've made your non-identified scroll work for you if the fists were beside the eyeball.
If it was Mirror Image, then this boss fight would be the time to read it, so the clones can lure the fists away from the eyeball & give the fists other targets, so you could make it to the eyeball safely, or at least attack one of the fists without getting hurt the first few turns.
If it was Challenge, then it would also do the job of luring the fists away from the eyeball.
The only thing that would be detrimental to use at the beginning of this boss fight would be if it's actually a Psionic Blast scroll.

I'm on 30 health so will have to waste another potion trying to get there, should I go for the P-Gas or use another potion?

No, you don't have to; simply step on a Sungrass seed & heal up out of sight, or during the fight against B-Fist.
See my next reply/explanation below.

...there's fire around there so I thought I could chuck a P-Gas potion in the general proximity of the fire and hope for the best, but I need to find a way to get within melee distance of it...

Judging from the screenshot you provided, you don't need to do anything but hide behind the nearest Decorated Wall tile, until the B-Fist approaches you.  (Moving 1 tile up from the position in that image.  If the B-Fist isn't going anywhere, let it see you, then hide behind the wall tile, until it approaches onto a water tile.)

You need to stay on a water tile when melee fighting the B-Fist, so its fire doesn't continually burn you.
(And if you can lure it onto a water tile, I think its fire-field will get doused.)
Also, you could plant a Sungrass seed on said water tile & stand on it for the duration of the fight against the B-Fist, so it can act as a heal-over-time item; though you'll still likely need to drink a Healing potion, or two, depending on how often the B-Fist damages you.
If you can, you can try to toss the P-Gas potion a few (~3-4) tiles past/behind the B-Fist during your melee fight, so that the P-Gas can expand to paralyze the B-Fist without also paralyzing you, allowing you to strike while it's paralyzed.

You won't have to use the Wand of Slowness until you're already fighting the B-Fist, if you take this approach, since you'll want it to approach you ASAP, without giving it time/distance to fire its ranged attack.  (NOTE:  After defeating the B-Fist, make sure you repeatedly tap the Wait button until you're fully healed from Herbal Healing, before stepping off & making your way to the eyeball.)

If executed properly, you should still have 3-2 Sungrass seeds left (depending on whether or not you healed up out of sight via Sungrass, before starting the fight), the 80%-filled Dew Vial, & maybe 1 Potion of Healing left, which you can use against the larvae while striking the eyeball.

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