Overall

1. Most ridiculous monster immunity in Hell difficulty removed, only kept a few reasonable ones as below.

Physical: Wraiths, Swarms, Night Marauders.

Magic: Unravellers, Wailing Beasts.

Fire: Burning Skeletons, including warriors, archers, mages.

Cold: Snow Yetis, Frozen Horrors.

Lightning: Will-o-Wisps, Flying Scimitars.

Poison: Machines, Structures, Flying Scimitars.

2. No immune monsters in normal difficulty, except for machines and structures, they’re always immune to poison.
3. No immune non-champion monsters in Nightmare difficulty, except for machines and structures, they’re always immune to poison.
4. Except for some ‘obviously should be’ ones, most champion monsters in Nightmare and Hell difficulty would have to own both ‘magic resist’ and a ‘enhanced’ MonUMod to acquire certain immunity.
5. Monster Resists Greatly Readjusted, basic rules as below~

Undead: Very high Poison resist, pretty high Elemental and Physical resists as well except Magic resist, Mostly, NOT all.

Demons: Notably higher Fire resist and zero Poison resist, Mostly, NOT all.

Wild Animals: Zero Fire resist, Mostly, NOT all.

Succubus: Just like Andariel, Very high Poison resist and Negative Fire resist.

Non-Undead Casters: Higher Magic resist, and Zero Physical resist, Mostly, NOT all.

Lumber and Botany: Negative Fire resist, Very high Poison resist and pretty high Cold resist, Mostly, NOT all.

Humanoid: Average All Elemental resists but Zero Poison resist, Mostly, NOT all.

Amphibious: Notably high Poison and Fire resist and Zero Cold resist, Mostly, NOT all.

Fire Theme Creatures: Notably high Fire resist and Zero Cold resist, Mostly, NOT all.

6. Guest monsters no longer appear in Act 5, instead, they replaced all the common monsters with the same names in Nightmare and Hell difficulties, also replaced champion monsters in Normal difficulty. This would be highly challenging.
7. Defense stat works on most ranged monsters’ missiles, including magical ones, even their CTCs.
8. Fulfilled more types of monsters’ ‘no freeze’, ‘stun attack’ or ‘steals MP’ descriptions.
9. Melee monsters attack more fluently in Nightmare and Hell difficulties, except those ‘Chargers’.
10. Monsters below are no longer ‘large’, so they could be knockback by Molten Boulder:

Minions, Bigheads, Flying Scimitars, Frog Demons, Vile Mothers, Fingermages.

11. Monsters below are set to ‘large’, so they couldn’t be knockback by ‘Molten Boulder’:

Balrogs.

12. Most monsters can’t be knocked back are set to ‘large’ so ‘Molten Boulder’ won’t roll over them:

Structures, Griswold, Machines such as Catapults, etc.

13. Simulate monsters below as ‘flat’, so they couldn’t be knockback by anything but being able to be rolled over by ‘Molten Boulder’ then suffer massive damage from it:

Sandmaggots, Baby Sandmaggots, Sandmaggot Queens, Swarms.

14. Monsters below are set to ‘shrunken’, won’t be able to steal HP or MP from them:

Wraith Series, Bone Fetish Series, Fingermage Series, Doom Knight Series, Thornhulk Series, Imp Series, Frozenhorror Series.

15. Simulate some monsters that should bleed green blood really bleed green:

Insects, Spiders, Regurgitators, Frogs, etc

16. Traps or Fire Towers and some structures are changed to 3rd party monsters, would attack everything, even each other.
17. Those extra solid Towers, Nests and Sarcophaguses now have an impressive chance to drop trinkets or runes, spending time to smash them would be more worthy.
18. When ‘Nests’ spawned as unique, their minions would be what they spawn instead of themselves, so there would be no longer a massive pack of Nests anymore, but blue Champion packs are still Nests.
19. Nests may not appear through random monster spawning, might spawn what they spawn instead.
20. All petite monsters spawn in huge packs had their HP and defense halved. This would make them easier to be killed by AOE skills. Beware, ‘Fallens’ are not included.
21. Most monsters no longer have their light radius.
22. Mana stealing monsters had their stealing values readjusted much