>>20079593After making this post, I played my very first game on the Library level, and won. I went for a completely different build this time, relying on crosses and clovers. It worked really well.
Getting higher luck helps SO MUCH! I managed to find 4 different attract-orbs sitting on the ground, which enabled me to jump a dozen levels in no time. After that, I just snowballed to victory. My boosted luck stat even enabled me to find a clover on the ground, boosting my luck stat even more.
The evolved form of the cross is actually pretty shitty. So in the late game I picked up the axe to give me some extra damage. And it worked out well.
Because I went Laurel and clover, I didn't have much damage output at the end. Thus I was barely able to hold back the final wave. But with my fully-upgraded Laurel I was damn near invincible anyway.
So there you go. The game is actually not difficult at all. I managed to beat these two levels on some of my first attempts. Without any meta-upgrades, using the basic-bitch intro character.