Thanks to all anons who helped my brainlet ass here >>545934886. I managed to clear it.
For future reference, I decently built Sexbell, poorly built Tabethe, and did not build Truffle.
Tabethe held about a dozen mobs, which allowed me to easily build combo and clear the kill requirement of the mob trials with great easy, a much better experience than before where I was many times getting stuck at Trial IV.
The boss still proved difficult and took me 3 resets, but that was mostly skill issue on my part as I had not used Lisbell up to this point. Basically, just had to make her sword very big and keep swinging it in the boss' general direction.
I fucking love her.