Much of it depends on your approach. Proper preparation can make any of them "not so much" a challenge. The Wither? Just make a Wither cage up in the Bedrock of the Nether, use a really good diamond sword, you can kill him off in seconds while he can't harm you. Woodland Mansion? If you set a bed respawn nearby and have proper gear, you can make short work of them. Even more so on an SMP server. Ocean Monument? Again, proper gear including slime blocks, redstone blocks, TNT for "shaped charges", pop that like a can of soda, call it done.
With the Ender Dragon, my wife and I would just use crap gear and a pickaxe to mine up stairs to the iron cages to break the End Crystals. Use bows to blow the rest. If we die and loose our gear? No sweat, really. Minecart chests for storing things. We may or may not use pumpkin heads as the mood suits us. Once the End Crystals are gone, we wail on the dragon until it's gone too. I'm sure there are even easier ways to handle the Dragon, but that's where we have our fun.
I'm not quite sure how to order difficulty. Your gear preparation and mental state make a huge difference.
If you want the challenge to be serious, you have to use something like Extra Hard Mode on a Spigot server. There may be a Forge version of it as well. Much of the preparation in Vanilla doesn't work in EHM and makes it far more a challenge. But EHM is a mod, and if you are playing Single Player it's much more difficult to set that EHM up.
Woodland mansion: hardest part is getting to one. It can easily be a 20k+ block trek. Mansion itself is peanuts. Bring a good bow (Power 5, Unbreaking 3, Infinity, Punch 2, Flame) and a stack of coal blocks for torches. Place itself is full of natural chokepoints to exploit. A typical underground ravine or mineshaft is harder to clear, IMHO.
End: again good bow and sword and diamond armor with Prot 4 on 3 pieces and Blast Protection 4 on 4th. Feather Fall 4 on boots. Bring several stacks of dirt for pillaring. Fairly trivial. The subsequent exploring and search for End Ships is a far more perilous endeavor.
Wither: hardest part is hunting for skulls. Fight itself is all about setup. Diamond armor 3xP4. 1xBP4. Sword with Smite 5. Powerful now just in case. Dug a tunnel at Y 40 away from other caves, a tiny 3x2x3 room to build Wither in. While it powers up, drank Regen2, Str2, Swift 2. And in their place on hotbar put splash healing 2. Ran in a chopped Wither up.
Ocean Monument: armor with Depth Strider 3 Respiration 3 and Aqua Affinity. Ender chest, at least one Shulker box filled with milk buckets. Lots of various blocks. I take Elytra out to ocean, drink waterbreathing potion, switch to chestpate above monument and dive bomb. Start by blocking up the little dome structure on top, fill with dirt as much as possible to displace water, take milk bucket, dig dirt out. Build nether portal for fast access. From there, it is easy to build out passages to wings and take out elders. Once elders are dead, the rest is trivial.
Woodland Mansions are a beginner's dungeon. They are very simple. If you can survive a whole night in the wilderness, you can easily clear one. With no trouble, I cleared one on a test world set on hard difficulty with just plain iron equipment, a non-enchanted bow, and a stack of arrows, torches, and potatoes. The problem is the "sight distance" of creepers, skeletons and the "new" vindicator and evoker mobs. You can see them down the long hallways and inside the rooms, but even when you pelt them with arrows, they can't see or react to you. They just stand there and let you shoot them until they die. Quite a letdown for those seeking any sort of challenge at all. Mojang failed here and failed even harder with the totem. Such a waste of great, interesting concepts that were poorly executed all around.
The Wither, unfortunately, is a breeze if one simply summons it below ground and prepares a little for the battle. There are guides that advise taking the fight to the Nether's ceiling, but that's completely unnecessary. Of course, equipment is a big determining factor as well; a max-enchant bow and max-enchant diamond equipment plus plenty of potions will turn down the difficulty knob to almost zero. If one summons it in the middle of a desert while wearing iron armor like I did the first time, you're in for a pretty dang good fight. Still, even then it has issues—first and foremost that Mojang is totally incompetent when it comes to mob AI. So, in my opinion, the Wither's difficulty is highly situational. It desperately needs an AI overhaul.
Ocean Monuments are also highly situational, but they're (thankfully) balanced for end game, so that's not so big of an issue. You either have max-enchant diamond gear with potions or you're not trying to clear one. Not much in-between. In my opinion, they're probably consistently the hardest thing in Minecraft. Mojang really got these right.
End Cities are, I think, supposed to be the greatest challenge of all and they can be pretty rough. Again, Mojang did a great job with these—they really nailed the End cities. In iron armor, shulkers do considerable damage and a fall from one of the weird structures usually means death. After you've cleared one and have the elytra, though, they aren't so tough. They drop below Ocean Monuments in difficulty at that point. It's much like working a maze backwards; a lot harder starting at the bottom and working your way up. With elytra, usually one starts at the top and doesn't even bother with the rest (unless farming shulker shells). Being levitated isn't so bad when you can fly. Check out my sig for a link to my first End City raid.
The Ender Dragon isn't very hard to those who have slain her a number of times before, but your first time can be a harrowing challenge! It's a really cool battle, though it needs some significant tweaks to really bring it alive. Again, (much) better AI would be highly beneficial.
So, the first time you defeat one of these challenges, it's a lot harder than other times. Your first Ocean Monument and End City raids can be pretty tough, but after that, not so much. Except the Woodland Mansions. They're just flat out easy due to terribly poor mob AI.
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Easiest challenge is definitely the mansion; using the well-known seed 455183159361683197, you spawn within sight of one at ~14 chunks render distance, and it has enchanted iron axes in chests on the first floor. I took out all the illagers in a few nights with no armour, no bow and arrow.
However, assuming you play the game straight, the mansion is only easy because it is not a strange environment. No water, no dimensions, no withering
The ocean monument is really easier than the mansion with the right tools; three potions of 8 minute water breathing, night vision, and strength are ideally all you need to just kill the elders and get the gold (well, plus at least iron sword and pick). Getting the sponges or doing anything else is harder though, because sponges mine slowly with all tools and even with enchantments, and guardians can be nasty in groups. Still, I'd rather take out a monument in 20 minutes than a mansion in two hours.
The enderdragon is simply a matter of how good your parcour is. If you bring water buckets, infinity bow or lots of arrows, a good sword, a strong pickaxe, lots of food, and maybe a magical item or two, you should be fine. It's harder than the other two, though, because the dragon has the environmental advantage this time. You need to be prepared for your strategy to fail because the dragon can outfly your running speed.
I have never defeated the Wither in survival no cheats because getting the skulls takes too long even with looting iii, but I'd guess it's the hardest. Sure, you can choose where to build it, but it eats or explodes everything it touches. The Nether ceiling is actually a better idea that I've never thought about. And it's a lot better than the underground overworld, because the Wither always tries to gain height when attacking, and denying it that with a nice thick layer of bedrock will make it struggle. You just have to have fire resistance potions, and protection v armour, so the wither effect can't hurt you.
Ocean Monuments[/b] are also highly situational, but they're (thankfully) balanced for end game, so that's not so big of an issue. You either have max-enchant diamond gear with potions or you're not trying to clear one. Not much in-between. In my opinion, they're probably consistently the hardest thing in Minecraft. Mojang really got these right.
Nah, ocean monuments can be cleared using just wood. Use trapdoors (wall filler that can be opened and closed as needed) and a couple doors (main egress point) to block off each room. A small room works quite well as an initial foothold/beachhead, giving you a safe spot to retreat to for storage, crafting, regrouping, resupplying, and just strategic maneuvering to deal with the guardians. With a bow and arrows, you can easily clear and defeat the entire monument without taking 1 point of damage from either the laser beams or from the thorns enchant.
The introduction of Drowned in 1.13 will significantly increase the danger level of a monument, though, to the point it'd just be plain suicidal to attempt without awesome gear AND the gold/sponges will be even less worth the effort.
Nah, ocean monuments can be cleared using just wood.
Sure. I've seen a number of tricks to defeat Ocean Monuments, the Wither, and the Ender Dragon. But, I wasn't including stuff like that. Or glitches. Or commands. Or switching to Peaceful/Creative mode.
I understand that 1.13 will also kill the "torch against the wall for air trick," too, which is a good thing. Anything to add at least some difficulty to this game.
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