Emc does introduce variety of mobs and each mobs have it's own hard to make challenge.
You can read mobs here
A stronger, more powerful mobs can beat player with iron armor easily if you're not focus. They can be easily recognized by having fire practice over them
These mobs auto-aggresive when set diff is 3. It can be disabled in player settings
Enraged zombie: Easiest mob in enraged section. Can easily beat with iron kits
Enraged skeleton/stray: Using bow (or bow with slowness effect for stray) offer higher damage. Beat easily if you have shield
Enraged creeper: Repeatly explode (sometime have werid glitch, make it not explode) which you can use for advantage. Loot usually reduced due to trigger LRD (low risk detection)
Enraged witch: Run, esecipally if you're new to emc without diamond armor. Posion 2/harming 2 can be extremely lethal for iron armor players
Enraged enderman: stay away, or at least if you can't run, build 3 block high up. It's dangerous for player those who wear iron armor. Also spawn in end (same rate as overworld) or nether (at much lower rate)
Enraged silverfish: Very annoying mob, it usually burns it's loots in fire. Leave a fire trails can burn any item on it's path. Spawn when mining ores
Enraged zombie pigman: Will not attack you if your diff is 8 and up
Enraged wither skeleton: usually spawn with marlix as assistant, but it can spawn rarely
Enraged ghast: More damage, health and it can slain player with iron armor easily with couple of hits. Just run if you don't have elytra or diamond armor (enchanted).
Enraged blaze: Just like regular blaze but higher damage and more health.
Enraged shulker: Drops great stuffs, but it can annoying due to leviation effect
These mobs are powerful, and take decent amount of time to beat. It can breach your armor quickly at diff 10, so just run especially if you have iron armor
Only auto-aggresive at diff 7 and up
Momentus:
It's annoying with momentus's guardian (easily as enraged zombie)
Easiest mobs in boss section, just repeatly hit it. No special effect
It can teleport at diff 7 to you or you have been teleported to him to prevent running away
Hit it as fast as you can (with cooldown)
Enchanted diamond armor can beat it at diff 1- (up to diff 9), or at least is starter armor at diff 1-(up to diff 5). Enchanted netherite armor can beat momentus at diff 10 with totem of undying, strength potion 2 if you constantly hit it quickly
Marlix:
Most hardest/annoying mobs. It just flying and you have low chance to beat it using bow/arrow
It left thing messy, tons of assistance with enraged wither skeletons/skeletons alike (with name change)
Can't drawn to you (aka teleport to you)
Can be stuck on object such as leaves due to it riding on invisible bat. You can quickly cover up to take it as advantage
Can't shoot it properly when on forest/taiga biome. Much easier stuck on leave/trees but have low chance to get itself out of stuck if you not trap it fast
Use at least diamond armor/netherite armor (enchanted) to beat it
It can slow you as well as shoot explosive arrow at random chance
Eyender:
Run. That's it. If you only have starter gear and without diamond enchanted armor at least
Able to slain you quickly if you lose concentrate, even on diff 5
Jump on water pond is recommend, as eyender are stuck on edge of area without water, you can take it as advantage.
Use charged pearl to beat it easily as it will double damage of weapon output, you can beat eyender's guardian to gain charged pearl
Able to perform amazing teleport to anywhere nearby to flip truss. Run back to water area is recommend (not 1 block water area)
Can save itself out of water
Sorigina:
Much lower spawn rate compared to other boss mobs
Extremely powerful/hard, just like marlix on flat/treeless biomes. Can easily slain you if you're not concentrate, even on diff 3
Sorigina's assistant is annoying, it's gift from sorigina.
Tons of fire and special potions such as slowness 2/harming 3/poison 2,.... All are for you
Very high health
Netherite armor (enchanted) is recommend/Diamond armor(enchanted) is minimum for beating sorigina
Best way to beat it is trapping sorigina into 1 space and constantly hit him. Don't kick him into water as he can teleport to out of water
These special mobs are dangerous, and it can be considered as sub-boss
Nether hounds:
Super aggressive to player
Calls horde alot and extremely annoying to dealt it
Tons of random effect like speed/strength make it more lethal and dangerous
Great rewards like gunpowder, quartz,....
Most major issues for player due it's dangerous and it can slain you easily when on iron armor or lose concentrate
Super turkey and blizzard:
Super easy, yeld rare stuffs like turkey slicer, turkey meat and diamonds/blizzard arms