Terraria - Arena design for events

I never really used arenas, but these are some suggestions. 1(Frost queen's icicles): You can put opened doors/trapdoors around the arena and close them when the icicles are used. Although on that case I would recommend the Ice Staff (the one that places temporary ice blocks.) 2(normal mobs getting in the way)Maybe make the arena in the air or use a Peace Candle (that might get in the way of the minibosses though) 3(Santa attacks)Try getting a black belt or the Eye Of Cthulhu shield, the Ice staff also works in this situation.

That's all the suggestions, I hope you make use of these and find an answer.

For the pumpkin moon, here is a very detailed video on creating an arena with some helpful advice that I have used myself. This one I have not tested, but it was made recently and in 1.4.

Create an arena with a lot of areas for enemies to spawn. Rows of long platforms, that go off the edge of the screen are best. Try to keep it away from caves, and other things that can get filled up with enemies. Obviously, also use a water candle and battle potion. Create a small box of shelter, or a covered minecart going side to side. Use a weapon like a storm bow, and you should be good.