I wouldn't say Supreme Helper Minion is tough. it's just time consuming. ...But, I think that is, because many approach that achievement wrongly. To begin with, many players don't fish at all. But once you actually realise that you can get a bunch of cool stuff from fishing, like frog leg, and hardmode ores from crates and such...
I think a good approach for supreme helper minion is simply to spread it to multiple playthrough. if you do 30 quests (golden fishing pole. guaranted reward at the 30th quest) if you're lucky, you might get the fish finder and the tackle bag too by that point. (though quite often not...) and then do a few more. 30~50 quests per playthrough. not 200 in a single playthrough.
Then you can get the achievement eventually. without getting super bored.
Because spending 200 consecutive in game days with fishing would be super boring indeed. but if approached in a different way, it's not that bad. just don't try to tackle it in one-go.
In fact, once you got the achievement, it feels weird when you fish in the next playthrough for the fish finder and stuff, and they won't count to the achievement anymore XD
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As for what is the toughest achievement though, I'm not sure. EoL is super difficult too. I managed to defeat her first time on expert just 2 days ago XD lol. But I still didn't managed to defeat expert moon lord, or have trouble with him even on normal mode, since his deathray passes through solid blocks...
Betsy is also tough, though managed to defeat her several times on normal. (it was before that achievement got added, so still don't have it XD lol)
Not an achievement per-se, but I think completing the bestiary is also very tough. didn't managed to do it so far... XD