After failing in 世界中の迷宮 for a few times, I finally managed to clear it.
Apparently the level boss, デネブ, would perform a one-hit kill if Stella takes too many turns during the battle.
So I use this configuration:
To ramp up STR. I could’ve also used トゲトゲの首輪 (chance of STR up during battles), but I use 警報の鈴 (avoid traps) because when there are lots of floors, Stella often gets hit by traps and the damage adds up ^^;
I think the general idea when tackling a level is to first only go halfway, equipping Stella with items to increase treasure finding. Then hope Stella gets the good weapons and armors from that level. Then equip Stella with those weapons and armors and a different set of items (no more treasure up, but give her more chance to survive to the last floor and defeat the boss) and go all the way to the last floor.