Okay, so, overall I like the poem. The structure of it is well-presented: they meet in their cover, brewing something, chanting spells, and presumably shrieking fear.

Disclaimer: In no way or form my critique is professional, although I try to be.

The first stanza can be a bit confusing. When I think of "wildfire", I think of the wildfire forests, not a 'wild fire' that you put under a cauldron. Secondly, the last line is an extra. You could split the third line into two, and it would have more power. Saying that it is boiling does not necessarily add value, especially since we expecting anything brewing/cooking in a cauldron to be boiling.

The second stanza is good, although I would put the last line as the second or third, and restructure it. As it is, two lines are short, two are longer, the flow isn't as even. Furthermore, always try to leave an impact with the last line, and I feel the other two lines do that better.

I'm not a huge fan of the third stanza. The first line seems that you had no idea what to say, so you wrote that. The 'cover of darkness', is a good description. I would enjoy more if you make the reader fear, why is the coven dreaded? I shouldn't have to think, oh it's a witch, spooky. The poem should be able to say, "If you say the witches, you would be so terrified that you would turn pale". Make me shiver. Make me bite my nails.

Having said that, it is still a good attempt, and I understand the constraints of lengths (and possibly rhymes) in contests/challenges make it harder. I, for one, am sure my poem could also use improvement. So, please, do not feel discouraged by my lengthy critique - if anything, be encouraged to continue to write and improve. That's the way of the poet.