The best arena is if you have teleporters on each end that connect to each other, powered by pressure plates when you stand on them. Those would make you able to run infinitely towards one direction, each time you teleport, enemies have to search for you again and restart attacking you so they will practically not be able to land any attacks, if you use weapons that automatically chase down enemies like the Razorblade Typhoon or Chlorophyte Bullets, you can just run and shoot and not even look at the screen.
My arena also has a roof of actuated solid blocks that protect me from attacks coming from the sky (Moon Lord and UFO's lasers) but it's not necessary if I am using the teleporters.
I also have traps for the Old One's Army event and an actuated lava floor then a dart machine. My all-purpose arena: https://i.imgur.com/lDXG7ZR.jpg My dart machine trap: https://i.imgur.com/8ISsLro.jpg My dart machine is aimed 2 blocks above the floor, aimed just at the head of most small enemies. My floor can be opened up to expose the lava but I suspect some enemies are immune to it, can't be sure, but the dart machine kills most of them. I only really need to worry about bosses.