Watch-Commander,
This is a truly complicated situation to fix! I’ll start with the artifact. If you found it in a ruin, as I did, you could try scavenging from similar ruins, although I would not necessarily expect success. If you purchased it, ask the merchant where she found it. If you made it, then create more? There aren’t very much shortcuts when it comes to artifact creation, not without very, very complicated infrastructure.
Onto the demons, then. First of all, can you negotiate with them? Demons are as complicated as humans, if not more so, any many varieties will accept bribes, trade, and even contract work.
Mendev appears to be a problem. If their internal corruption is making them less use as an ally, then make preparations to take over their power structures. Wait until the demons kill them (assuming they are indeed focusing on them), sweep in, and take over their infrastructure and resources. This will let you use them more efficiently.
As for the combat issues, I’m afraid you are likely more the expert than I am. Do certain troops do better than others? See if they can be scaled up, etc. If manpower is an issue, focus on propaganda and recruitment. If specifically powerful individuals will be more useful, use your resources to recruit more sorcerers and Dragonblooded. Titles often make extremely effective bribes, and require little upfront cost. They’re inconvenient longterm, but I expect that is not a current concern. Some may even be persuaded by the fact that they live on the planet, and thus the apocalypse would be bad for them!
The situation in general seems somewhat suspicious, now that I think of it. It’s possible that the demons are being created in-creation by a first or second circle, rather than traveling from Hell (you have not mentioned logistics issues on their end, and that would be an obvious target if they were crossing The Endless Desert). In that case, assassination might be worthwhile, although I would not precisely expect it to work.
If you haven’t petitioned Heaven for aid yet, do so. They probably won’t respond within the century, but it’s worthwhile to make the attempt. Invoke their duties as enemies of the Yozi, point out the risks to Creation, and threaten to audit them.
Demons are incredibly varied creatures, and have no weakness that applies to all species. Should you know what species you are dealing with, or even their creator, I could provide more specific information.
I assume you are using thaumaturgy? Collaborate with your allies for techniques, and see if they can be combined. Use the most effective rituals from every culture.
….It really just sound like you need sorcerers, though.