Yeaaah, I think they're just really rare. This is my story: Make new world on 1.3 (Expert/large, not that I think it really matters), progress pretty far (beat BoC and EoC), never found a Strange Plant until I found one on a Sky Island, maybe 5-10 hours of gameplay in. Traded it in to the dyeguy and got Reflective Copper Dye. I used it, it was interesting because it was reflective but it wasn't a very pretty dye, I'd still prefer pink (although cactus fruit are way too rare).
Then an hour or two later I went back up onto the island and found another one with a different sprite, in nearly the exact same spot, give or take a few tiles. Traded it in and got Reflective Dye, even worse than the copper one.
Then approximately four to five hours later I went back to the island and found a third Strange Plant, yet again with a different sprite. Traded it in and this time I got Acid Dye, which is a
ing beautiful dye. I was too lazy to figure out how to make a GIF out of it but below you see that it's like a congealment of different shades of green, and in-game it actually has a really cool animation where it like changes color in waves of different green and
.
Anyway I've put like 10-20 hours into that same world and haven't found another Strange Plant since. But I did make a handful of new worlds and ran across the surface looking for more but never found any. But yesterday I made a new hardcore character and a new world (expert, small) and found one maybe 200 blocks east of spawn in Forest biome. So yeah my hunch is that they just spawn wherever foliage does, just at a super low rate, and maybe for some reason they have a higher spawn rate on sky islands.
I found one strange plant at the start of the game before I even had any idea what it was, I found it in a cave at the surface right by my spawn point. I haven't seen a single one since then though.
I had the exact same problem, but once I tried using a featherfall potion I (just barely) managed to beat him on my first try.
Yeah, in expert mode you HAVE TO destroy his hands first, otherwise you're screwed.