Dodging the Skeletron himself is the main thing here IMO. Cus if he manages to touch you while spinning it most likely means a stunlock leading to death. Dodging hands is by far not as important and usually simple jumping is enough. Some HP (like 300+) makes things easier as well.
My arena served two purposes (and wasn't even a straight line and had no additional layers of platform whatsoever) so had two properties - to be long enough to be able to run away from spinning Skeletron (about 5 screens wide turned out to be more than enough, 4 campfires did cover it all) and to have small "shields" (like 11 blocks wide with a hole in the middle to be able to shoot from under it) above it in some points, mostly on sides.
All gear I needed was a Space Gun (with Meteor set ofc), Shield of Cthulhu, some Spectre+ boots, something crafted from Lucky Horseshoe and Balloon(s), and two more random accessories (ah, one of them was Band of Regeneration I guess). Ah, healing potion was used once.
The fight itself was pretty standard, as many people described already. Get to one side of the arena somehow and start shooting hands, prioritizing the one with lower hp. You can slowly run while doing it, mostly after jumping over one of the charging hands. Try keeping a little distance from head to allow easier side charge to start evasive maneuver after head starts spinning. Aim is not that important, DPS is not an issue here. Try to take it easy, don't be too focused, you'll need that focus in the second stage. Don't shoot the almost dead hand if second is not close to death as well. Ideally you should shoot em both down during head's his spin phase. So after hands are down rush under one of "shields" on either side of the arena. Take aim up into the hole and keep firing, head'll fly right above you eventually. Now the most important part - evade his spin nicely, aka double tap and keep running (while still shooting at him) when Skeletron suddenly makes that scream and starts spinning into you. You can even count to 10-11 slowly while standing under shield while shooting and start running preemptively, takes a little practice. But I highly recommend starting the run a little before he charges, that takes a lot less concentration and actually reacting withing less than a second every single time with no fails for like 10-15 times might be a little hard. Risk of getting hit (you can jump over anyway) by a homing skill (or even a couple) every 20 seconds in this fight is negligible compared to getting hit by a spinning head. I repeat - if you get hit by a spinning head it's most likely over (you can try getting out with a hook and stuff, but that takes even more skill). Repeat this "thru-hole shooting while counting and then rushing to the other side of the arena" thing until he dies.
