I struggled with this and found some tips and tricks here and there, wanted to share what worked for me without needing gold gear or forest amulet. The strategy here is to stun-lock the Witch as often as possible, while having the possibility to heal when needed,
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2995013860

It's fairly simple, you need 4 hammers (stun) and 3 magic staffs (heal). Iron shields for all (extra 10% stun chance) and wolf heads for all (good defense), which are farmable killing wolves, buying the armory card pack (15c).




You can farm mobs that drop it, or open chests.
I find that this slime gives the most magic dust as it splits into smaller parts, which you can farm using the same Armory card pack (15c).




If you don't have these unlocked, keep buying card packs to discover more ideas.
It's also recommended to flee the dark forest if your units have low health, you will resume the same wave on your next run into it.

Of course, if you have better gear that you may have gotten from killing the demon, or other strong foe, use them instead! (if they have higher chances of stun or heal).