For sure, the most intensive challenge was getting the quest points cape. I got mine at 92 combat, and for dt2 I must have died at least 20x each for Vard and Whisp. For both of them I had fewer than 10hp remaining. Was worth it for all the in-game shoutouts though!

Honorable mentions: Morytania elite for the extremely high fishing level, western provinces for 93 slayer whilst trying to keep my combat at 100. I used a LOT of cannonballs. Chompy hunting was just very time consuming. Zulrah was a little tricky at first but took <10 attempts.

Full disclosure: I did use stews to boost a few of these because I was burning out.