I have been grinding 1-11 since yesterday to get those UC weapons.
The minmaxer in me doesnt want it because it has sub optimal stats but for now I have no choice, After beating 1-9 it was smooth sailing until 1-15 which is kinda hard if you dont kite the monsters very well, then I hit a wall on 1-17, hopefully with another UC weapon Ill be able to clear it, I do not reccomend farming 1-15 until you get more power, since you will most likely need more sand and 1-15 has more difficulty.
On another note i also tried the "enigmatic heart" Abyss and couldnt clear it, not even close with 12400 power, I'll try again when i have at least 14000.
Here is a video of the treant fight, you have to position your ranged characters properly and avoid if needed.