For Lv. 10, it looks like the worst parts are probably Goemon (just for the 100k first turn attack and the preemptive 75% damage cut for his 27 million HP) and Awoken Karin(who all her attacks deal 55k+ and she has 51 million HP and 500k defense), though one must be sure to have an answer to Beelze's poison board as well. (and he hits hard too, 45k, 55k(under half), and 67k(every other turn))
I managed to blindly claw my way past Goemon with Reincarnated Karin without looking things up(took me a while to get past Titania but that's a player skill issue more than anything) but ran out of juice to punch through enemy!Karin's defenses/resist.