Normal, magical, rare and unique mobs sustain balancing

Normal mobs, are very fragile and effortless to shred through.

Magic mobs, are not a major threat but sustain more and last longer in fights.

Rare mobs, some have have an acceptable amount of sustain, on the other hand some are very tanky and require you to chip away at them little by little.

Unique mobs, like the giant killer ant in the interlude, running you down at max speed, doing massive amounts of damage and taking a long time to whittle down.

what are your thoughts on balancing the sustain of normal, magical and rare mobs?
-Normal mobs become slightly tankier.
-Magical mobs remain the same.
-Rare mobs become slightly less tanky.
-Unique mobs could remain the same because they are like a memorable menace, their amount of drops should be high.

Resistances for mobs, instead of having a flat elemental resistance, only those that wield the power of ice for example may have resistance to cold.
In no case should a rare melee mob with extreme movement speeds also have a slowing AOE passive.

Instead of gaining a passive that can be copied between legendary mobs for example aura effects, imagine some legendary mobs may have a unique skill. while a legendary wasp for example could have +50% attack speed.

in my experience-
In most cases the legendary mob is the last one standing, at times the gameplay in that window of defeating the monster can be repetitive, dodging and attacking. Especially the Eternal Knight mobs, the gameplay can become very silly, dodging its skulls and slowly whittling them down. Their skulls also do a large amount of damage,

please share your feedback and opinions about the differences in sustain between mobs rarity.