It's not that the fights are hard, but when you have limited playing time, and the upgrades can be such a pain to get, I think it's worth paying for some of them, but in the meantime, working on your own pops.
My advice would be to farm souls alongside plates + horns, always break off and kill T2 + T3 when you can then get back to plates/horns in the downtime.
So when you inevitably only have souls left, you should hopefully have some decent progress already.
Sellers can rate me down, but as of the 99cap a BST can easily solo the Grauberg T2 and T3 with a proper setup. Paying through the nose for these makes about as much sense as paying for EXP.