It sounds like you have the right idea, but here's some tips or suggestions. Buff potions definitely help, and also you will want to make sure you are using the Greater Health Potions, not the lesser ones of course.
If you haven't done so already, I suggest you try making a very long bridge. It doesn't have to be completely flat but at least enough that it's very easily traverse-able and you won't get stuck somewhere by accident or fall into lava. You'll want to focus more on the battle than trying to get around down there.
My weapon of choice for this battle would be one of the flails (Or any big Sword such as Night's Edge) for the numerous mini enemies attached to the wall. Flails take them out easily and quite efficiently since you can hit almost all of them at once. Once those are all done for, this is when switching to the minishark is a good idea. It puts out lots of damage very quickly, obviously, plus the bonus ability to save ammo every-time you shoot it. If you have the Goblin Tinkerer I suggest you put some money in reforging weapons for better stats, not necessary but makes a very noticeable impact when you get a good one.
As for your accessories, you might want rocket/specter boots to get around easier; a cobalt/obsidian shield so you aren't knocked around or into lava, and also in case there any exposed hellstone you could accidentally walk onto; and a Band of Regeneration to quickly recover lost HP. The other two are up to your preference really. I would also suggest reforging these to try and get the Warding prefix (+4 defense). Again not necessary but it will help tremendously in the long run. Especially since having 5 warding accessories means +20 defense.
You can also try to focus on magic items to defeat it. A Vilethorn also makes quick work of the smaller enemies while a space gun (with full meteor armor) will constantly damage the actual Wall without ever using mana. A Demon Scythe is definitely my favorite weapon to use against it though. It will hit everything it passes through, and while it's "charging up", the boss can be directly over it getting hit 3-5 times guaranteed. The only problem is how hard is can sometimes be to find.
If you do decide to use magic items make sure you have a full 200 Max Mana and some Mana potions as well. Restoration potions can also help here as they give back both mana and HP but will give the "Potion Sickness" debuff on use, unlike regular Mana potions. You can also get a Mana Flower to equip. This will automatically use mana potions for you so that you won't have to worry about how much mana you currently have.
Once you do manage to defeat it the first time, beating it again will just get easier since you will have access to higher tier items.
According to the official Terraria wiki, you should avoid getting too close to the dungeon from underground, if you haven't defeated Skeletron yet.
Deep caves and Chasms near the Dungeon should be avoided until Skeletron is defeated, as having a significant portion of the Dungeon on-screen below zero depth is considered being inside it, and Guardians can therefore still spawn.
This means that you do not need to actually be inside the dungeon for the Dungeon Guardian to spawn. The reason for this, is because the Dungeon is not a closed space; in my world for instance, the dungeon was generated with a patch of dirt separating two patches of wall, so I could've entered the dungeon without actually digging through its wall.
Best Answer
Lawtonfogle's answer works if you have two people, but if you want to do it solo, there are two ways:Note: Lawtonfogle's answer no longer works as of patch 1.2.0.3.Use Turtle Armor and Black Belt, and there is a chance the Black Belt will let you dodge the first hit, while still bouncing back damage. Then on the second hit, you will die, but the Guardian will die too. (Of course, there's a very small chance that you'll survive two hits, leaving you unharmed and one Guardian dead.)Note: no longer works in 1.2.0.3. The Guardians now only do 1000 damage, so it takes 10 bounce-backs to kill them.source