Terraria: Fighting the King Slime

I don't know why, but I had the most trouble with arguably the easiest (or second easiest) boss in the game! I got immediately destroyed the first time I fought it, halfway into pre-hardmode (the only time I failed to defeat a boss the first time around). The second time, here, was when I was in early/mid hardmode and I still almost died!! It's teleportation really cucked me over lol