I've seen many other Dungeon Guardian guides here, but none are super complete. Some are mere "suggestions", while others aren't super comprehensive. If you're offended by this, let me know with constructive feedback. But ultimately, it's up to the mods to decide. Mods, if you feel that this is a redundant thing, than by all means lock it.
But if not, then I'll provide a very complete guide tailored to your class.
Say you've beaten every boss in mobile Terraria. What now? You have all of this great equipment, but what do you use it for? Fear not Terrarians, for there is another challenge. The supposedly "unbeatable" Dungeon Guardian.
Of course, there are always hacks to beat it. You can use turtle armor. You can make a worldwide asphalt skybridge. You can use 9999 heart arrows. But ultimately, these methods are a bit cheesy. If you're a person who likes to cheese bosses, then I'm fine with that. To each his own. There's really no cheating in this game unless you mess with the code.
But this is a guide for people who have the gumption to face the Dungeon Guardian with nothing but their weapons and skill. Or those people who are too lazy to build a cheesy arena.
So, here, presented to you, is a cheesless way of defeating that unbeatable enemy. You may die. Scratch that, you'll most definitely die. A lot. But you will beat it. I did. And it will be worth it. So without further ado.
First of all, what even is the Dungeon Guardian?
The Dungeon Guardian is a supposedly invincible mob intended to keep people out of the Dungeon before defeating Skeletron. It's usually pretty effective too.
It's often regarded as the "true final boss" of Terraria. Take a look at the stat breakdown, and you'll see why:
Damage: 1000 nm/2000 em
Health: 9999 nm/19998 em
Knockback Resistance: 100%
What exactly does this thing drop? It drops the bone key. Which summons a pet. It's not very useful, but you get VERY neat bragging rights.
Now you could've gotten all of this from the wiki. What I'm here to do, is tell you how to beat it.
Of course, half the battle is preparing. Preparation is vital to your success. If you take my advice listed here, than you'll probably slay the giant skull. But if not. failing to prepare is preparing to fail my friend. First of all, you need to know a few things about this fight. One, no matter what weapon you'll use, you only ever do 1 damage a hit. 2 if you're lucky. The reason for this is that it has as much defense as health. So when choosing a weapon to take on the Dungeon Guardian, always opt for firing rate over damage.
First thing's first, every good fight starts with preparation. Equipment is the first thing you should take into account. First, equipment that you'll need regardless of class, then, equipment tailored to your preferred class. Note that with my recommendations, you'll still have room for some of your favorite accessories.
For Spectre Armor, you can choose either. But lets be honest here. 1000 damage a hit in normal mode, healing won't matter much. 9999 defense, extra damage won't matter much either. So you could choose either, but neither will really matter in this fight. Mana Flower will basically guarantee you unlimited mana, and the debuff really doesn't matter. Unlimited mana rules out over damage. Everything overrules damage here. I chose Razorpine for it's firing speed.
For melee, there aren't really a huge amount of super fast weapons. The fastest melee weapon is the Fetid Baghnakhs. But it's unrealistic to fight the DG with those. Paladin's Hammer is fast, but has limited range. Vampire Knives are a good choice; the extra projectiles will do some extra damage. Terra Blade is okay, but with 1 damage a hit, then you'll only be able to kill it as fast as you can swing it. There are better choices. Flairon and Scourge of the Corrupter are probably your best choices for melee. Flairon because of the bubbles, and Scourge of the Corruptor for the extra little eaters. Really, you could use any weapon that makes extra projectiles, such as North Pole. It all comes down to what you prefer. With Beetle Shell armor, and a few defensive accessories, It's actually possible to survive a hit by the DG.
The Chain Gun is the fastest ranged weapon in mobile. However, it's hugely inaccurate. Crystal Bullets, since they split, will cut down some of your bullet usage. But you're going to miss. So yes, over 10,000 bullets is a rational number. It would be a huge disappointment to run out of bullets in the middle of your fight. Notice that I recommend the Shroomite Mask for it's bullet bonus. But as I mentioned before, damage doesn't matter. So you could get away with any head piece, but I chose the Mask for completeness' sake.
Ok summoners, your turn. I really wouldn't recommend using a summoner build on this. You can do damage faster with another build, but if you're bound and determined, who am I to stop you? Also, your damage will ramp up quite a bit if you use the chain gun alongside your summons (credit goes to @Omega Derpling for that idea). Note that you'll need far more bullets for this since there's no ammo use reduction, but it will dramatically increase your chances. I chose ravens because 1) they fly, and 2) they're pretty fast. I chose Tiki over Spooky because (you're getting tired of hearing me say this), damage does't matter. A higher number of units will do more damage to the DG than the damage bonus of Spooky will do. Also, you can choose either Necromantic Scroll or Papyrus Scarab because they both summon one more unit. The damage bonus is negligible. Then, of course, you'll need as many summoner potions that you can take, that one extra unit will pay off. 15 will buy you 90 minutes of summoner time, if you time it right. This may or may not be enough, but it's always best to err on the side of caution. Note that with all of the movement accessories, you won't have enough room to include one accessory. Pick one you'd like to sacrifice, I would recommend the pigmy necklace, but you must use your discretion.
All of the weapon modifiers are only designed to increase your critical strike chance. Godly, Demonic, and Zealous are all alternatives to the recommended modifiers, since all we're looking for is that +5% increased critical strike chance.
Next we should talk about your arena.
You should build it in a brand-new world, with a brand-new dungeon. Obviously, because if you killed Skeletron, then the DG won't spawn. On your new dungeon, you first need to destroy the roof. Nothing need stop you from flying out of there.
You'll want to make a skybridge as well. I've seen many people suggest an asphalt skybridge. This isn't a bad idea, but it doesn't actually work. First of all, if you're wearing Lighting/Frostspark boots, then asphalt has a tendency to not work on mobile. Second, you outrun the DG on asphalt. This should not happen. You must keep that giant skull in your sight while on the skybridge. The DG should slowly gain on you, and you should be able to make it to the end of your skybridge. Then, you'll be able to easily outrun it with wings. So you could use asphalt, but I wouldn't recommend it.
Your second step is building this skybridge. Make sure that it's not too high, as you don't want wyverns and harpies spawning. Also make sure that it's not right above the dungeon. Make something like this:
So make one edge of the skybridge near the dungeon, but not hanging over. You'll thank yourself when you have to fly up to it.
Now, you don't need to make it too long either. But it's better too long than too short. It should be just long enough that the DG will be able to catch you as you run along it.
Don't bother with buffs like heart lanterns and campfires. They won't help. You'll want to make a straight shot from the dungeon up to your skybridge. It's a good idea to dig straight down to where the DG will spawn (trust me, you'll know), so you have a clear path of escape. You may die some, so you'll either want a bed at the dungeon, or put a teleporter from your spawn point.
So, you've gathered the right equipment. You've built the insanely simple "arena", and you're ready to fight. How do you do this? Well, here is a sequenced list of steps for the "fun part" of the battle.
Step 1: "Summoning" the Dungeon Guardian
Slowly, very slowly, cautiously walk down into the dungeon. One. Block. At. A. Time. (Platforms help) Make sure that your tunnel is well lit. You'll see the Dungeon Guardian coming at you. You'll plead for mercy. Then you'll get the heck out of there. As soon as you see that terrifying image of death, start flying. Hopefully, your wings and accessories are enough to outrun it. They were for me. Fly straight up through your tunnel, and don't stop until you reach the skybridge. It will follow you. Beware that you might summon multiple. You only need to kill one.
Step 2: The Sequence of the Fight.
By the time your reach your skybridge, you'll have far outpaced your adversary. So you'll have to wait for him. Don't worry, he'll come. As soon as you see him at the edge of your screen, start running. Once your boots kick in, he'll only be slightly faster than you. As you're running from him, start shooting. Do damage, as much as you can, while you're running. As I mentioned before, he's slightly faster than you. He'll slowly gain. Keep going until he's mere inches from you. Then fly up. As I said before, you fly much faster than he does. So you fly up, over, and around your opponent like so:
This is your basic flight plan. Run, fly up and around, then run again.
What this will do is put him on the other side of you, with you running the other way. Shoot again. Inevitably, he'll catch up to you. Then you'll have to do your little flight trick again. Magic users and Melee users can probably shoot it while flying. For ranged users, you'll most likely miss. Don't attempt it. It'll only make you run out of bullets sooner. Summoners, of course, don't have a choice in the matter.
Step 3: The Boring Part
You'll need to run, fly, rinse, and repeat for a really long time. With 9999 health and the same defense, It'll take you a while. While this part can get very boring, and very monotonous, you must never let your guard down. Nothing would be worse than having it almost dead, then dying.
Step 4: The Victory
If multiple Dungeon Guardian's were summoned, then you'll die here. After killing one of them, collect your loot and let yourself die. You got what you came for. Congratulations.
I hope you enjoyed this guide, and I hope you feel the same sense of accomplishment that I did when I defeated it. Tell if it worked in the comments, or give me feedback.
Thank you for your time!