You don't have to rely solely on random runes, they boost your performance in one way or another (different chest types boost different aspects of your performance). I would say combining all this together will grant you a huge advantage against bigger enemies, there are some playthroughs where players defeated bosses in a matter of a few seconds, which looks really cool even from our perspective, where we spend lots of time testing and trying different approaches. Alternatively, there are amulets that grant infinity attacks that have also an impact, you can charge your ultimate broom ability by defeating corruption eyes that spawn in the boss location and pick up more companions that also can significantly increase your performance against tougher enemies. If you will just pick up some alternative spells, and save just one or two for the boss fight, you can defeat it pretty easily, you can switch and choose your spells so it should be no problem, also there are plenty of them on every single level.

Let me just pull out the pressure, and say it is totally fine to refund or don't like the game, we are just having a discussion and why would I even think about deleting the thread if we are having a healthy talk? Though it is not very nice stating that someone talks stupid or BS if you are not taking the same position, I'm not offended so no worries about it.

Instaling Bosorka