This seems like a fairly simple question of economics... ok, simple for economics at least.
Your Potion Guild is trying to place themselves as the primary supplier of useful potions: healing, mana and stamina potions at least, maybe some more interesting utility potions like invisibility or ability buffs. They're using modern alchemical techniques to produce potions that, while not necessarily of peak potency, are at least consistent and safe. They found that good alchemical hygiene practices - like cleaning your tools not just physically but magically - result not just in more consistent results but much higher potion purity, vastly reducing the side effects and toxicity of their potions.
In other words they've got a good product, they just need to get the production costs down to the point where they can make a reasonable profit.
Depending on how alchemy actually works in your setting the mass production of potions may be as simple as scaling up the process. Can you run off a thousand doses in a huge vat or must each potion be crafted in a small cauldron? Do you have to infuse magic into the brew, and if so can teams of alchemists provide a steady stream or does it have to be a single person? How much of the process can be handled by apprentices and how much requires the skills of a master alchemist?
I'm going to assume that you've got all that covered and move on to the fun stuff: building demand.
This moves a little out of economics and more into marketing. You've got a good product, you just need to make people aware of it. Sure, you can sell it in markets and so forth, rent a shop in the merchant district, all that fun stuff. But that's far too slow.
During the initial phases you'll need to capture the attention of your target market as quickly and as solidly as possible. You need a brand, something that stands out amongst the crowd of inferior products. You need to get that brand in front of your potential purchasers and show them how your product is going to improve their existence. And hey, that's not even a lie!
You'll want to invest in distinctive, single-use potion containers that can't just be refilled with inferior product. Something that is partially destroyed on use would be fantastic, especially if it's cheap and easy to stamp a colorful logo on. These containers will be the symbol of your product wherever they're seen, standing out from the haphazard collection of bottles, jars and pouches other potions come in.
Getting them in front of the target market could be a simple as handing out some testers to the adventuring guild, promising them a small discount on the potions when they buy in bulk. Once the guild sees how good your product is they're going to want those potions anyway.
But let me pitch you a classic marketing ploy: endorsements! Go out and find the most powerful, most charismatic adventurers in the area and promise them discounts, preferential treatment and outright payment (as a last resort of course) if they'll openly carry your potions on their person. They get a good deal on life-preserving potions, you get even greater brand recognition. And you have potent adventurers who are now keen on preserving your business against overt aggression from disgruntled witches. Bonus!
As production ramps up, enter into some supply deals with the local guards, the nobility and eventually the army. The higher you can reach the more product you can move, and the higher you profits become. And when you're supplying product to the top layers of society you can use that to encourage more sales at the mid and lower levels. In much the same way as endorsements will have lower-ranked adventurers flocking to buy your product, a simple "as used by Lord Bob's elite troops" sign will do wonders for your sales. Trust me.
None of the above is even revolutionary these days, and there are a ton of other things that a half-baked marketing exec would throw in for a product launch. The fact that you have an actually good product would have half of the marketing professionals of today begging to work with you.
In a high fantasy environment however a lot of this is new. There should already be examples of most of it - except maybe the celebrity endorsements - in the day-to-day operation of large merchant houses. All it takes is a little vision... and the prospect of making a lot of profit will invoke a lot of visions in any merchant worth the title.
While the economic stuff is important to solidifying your guild's position, having powerful protection is going to be even more important in the early days. Building relationships with people and groups who can help protect you from retaliation is a must. Get the local adventurers on-side, then the guards, then the military. Let them decide for themselves that your business is too important to them for anyone else to interfere. Hell, bribe some officials if you have to. Build some less public relationships with the seedy side as well, just in case. It's hard to hire assassins to destroy the source of their favorite potions after all... and having some specialist negotiators to 'dissuade' the competition might come in handy.