Regarding the nurse:
You can actually keep her about 99% safe throughout the battle. Make a spawn point about, mmmm, 6-7 screens from the main arena, and make sure the path between them is clear. Pave the whole thing in adamantite if you need to, just keep it clear and smooth. Use the magic mirror midfight to teleport to her, heal up, then run back to the arena. Worst case, you run into the boss you're fighting halfway there and have to dodge a bit getting back in.
Edit: I think I broke my world.
It started with an attempt to flood hell. I've noticed that a few of my water drops last for a few minutes down there, so the plan was to make a series of massive reservoirs, separated with active stone blocks, and a 50 pump water duplicator at the top, to continuously drop large chunks of water down there.
First oddity was that my initial reservoir didn't drain at all through the 2 block hole in the bottom. Well, not quite right. It drained, but it never emptied, not a square, without the pumps running. Now, the water getting close to hell in my screen wide, all the way to hell watertrap is starting to stack funnily. Its not flowing, water is being treated like dirt by itself and flowing around it.
alright, I died due to explosive's and drowning, and my tank emptied out instantly. Good news, there is a lot of water in hell, not enough, but a lot. So I might do this.