The enemies here use a lot of magic, including Frizz, Sizz, Fuddle, and Kafizzle.
Since it's fixed damage, it will reduce your HP regardless of your Defense. Healing is almost a necessity.
Additionally, the overall enemy agility is high, so it might be easier to attack them with magic.
Among the enemies, you need to be especially careful of Mimic. Not only does it have very high Attack, but it can also use the instant-death magic Whack.
Whack has a 1/5 chance of wiping out the target regardless of their remaining HP. It's extremely dangerous when cast, so try to defeat Mimic as quickly as possible.
The boss here uses Frizzle, which deals a fixed 70 damage. You'll have a tough time if your HP is below 140. Healing magic is absolutely essential.
Furthermore, the side-boss Prestidigitator will cast magic like Frizz, Sizz, and Dazzle, while the MagicSlime casts Fuddle. Be especially careful of Fuddle, as it will make it very hard for you to heal once it's cast on you.
The easiest way to defeat them is to take them down in this order: MagicSlime → MageOfDarkness → Prestidigitator. If you're confident you won't get hit by Fuddle, you can also go MageOfDarkness → Prestidigitator → MagicSlime.