Challenge mode question

[NEStefan]

So I decided to 100% the game and gave challenge mode a try. Thing is I equipped an iron heart but still die in 1 hit, likely I'm missing something or are the rules of challenge mode different? Didn't find any place that has the EXACT rules for challenge mode.

[NEStefan]

equipped a second one and now it works for both. wonder why it didn't work for the first one but oh well.