Uhm, I don't know why anyone is having trouble with Hallow/Corruption infesting their base, unless you're using rough stone as your building material. I used nothing but grey brick and grey brick walls, and it stops both dead. I've got Hallow up to the edges of my walls (above and below ground) in my base, but I can confirm that neither that nor Corruption (from Corrupters) will spread to grey brick. I just set up simple entrance blockers with walls of dart shooters and a bit of active stone that I can open from either side, and absolutely everything, including unicorns, will kill itself trying to get in. Since I'm buried, I don't have to worry about wyvern spawns, and since I Hallowified everything around me, I don't have to deal with wraiths or worms. I have yet to have a single enemy get into spawn, apart from a Destroyer that I pulled too close to home.
For that matter, the Wall isn't exactly a big challenge either. Just pave over a 1000-1500 block highway in hell with a lava pool at one end, the bottom of your hellivator at the other, and backstops at both to prevent yourself from overrrunning it. All you need is a set of Hermes boots and some sort of ranged weapon, and you're set. I did my first Wall with starcannon and molten, but didn't bother with it after that. You can very easily do it with a minishark and necro, so long as you have Hermes boots. I actually never even saw the Wall more than twice in a total of around 20 kills on it, because I was always a couple screens ahead of it. The two times were when I wanted to see what it actually looked like. I don't recall ever getting a kill later than about halfway-2/3 of the way down the runway, either, and never used or needed a single health potion or needed combat drugs.
Similarly, if you set up a proper skybridge (maybe 2500 tiles long) not very high off the ground, get Spectre boots and wings, and get a minishark with about 1.5k silver bullets, you can easily take down the Destroyer a couple times, get the souls to make a megashark, and start killing Skelly Prime. By the time you take him on, you should have a mythril set. Once you have a flamethrower and megashark, just grind to full adamantite, and then start grinding the Twins and the other two until you have enough souls for all the Hallowed gear. Seriously, all you need are a skybridge, Spec. boots + wings, and the drive to upgrade your weapons and armor, plus some combat drugs and you easily have the capability to solo the hardmode bosses. They all have very simple patterns (possibly even to the same degree as easymode bosses), and once you memorize those and figure out how to dodge them, you can take them down multiple times per night. Just start with the Destroyer and Cobalt armor + minishark/Clockwork AR and upgrade your armor each time you progress to a new boss, and it is very simple, and in fact very similar to the pre-hardmode progression of metal>dark/shadow>meteor>jungle>dungeon>hell (prewall>wall weapons>cobalt + wall weapons>mythril + megashark, etc.>adamantite + megashark & flamethrower>Hallowed)
The main thing to keep in mind is that hardmode is very ranged-centric, as the megashark and flamethrower, from what I've seen thusfar, are not only safest, but are also the most reliable, due to their high degree of accuracy, velocity, and DPS. The star cannon is too low-velocity to keep up with post-wall bosses, wall and pre-wall weapons aren't high enough damage once you get better stuff, melee is too dangerous for bosses and strong mobs, and magic tends to be hard-limited by mana (I tried soloing bosses with magic-oriented gear; even with max mana, low use, and good regen, I would repeatedly run out and have to chug potions while also putting out lower DPS than with ranged gear). One last thing: When using the megashark, use the cursed bullets, as they not only get the additional damage for the green flame debuff, but it also lights enemies up well in the darkness. My Skelly Prime battles typically have all four of his limbs on fire for the whole time. Oh, and that is one last thing: With Skeletron Prime, not only should you intentionally take down his arms first to make things much easier, but you will accidentally do so if you keep attacking his head. For me, at least, they will always go down in this order: Laser arm first, then bomb arm, then drill/saw arm, and then the head, as the clamp arm is slow and sucks ass.
Wow, that ended up being a bit of a Wall...
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