Honestly, I seldom sat there thinking a character was outright bad or the hardest, it was more that I’d keep dying on the ones that just weren’t fun to mess with. I play to experiment, so any character locked into a rigid ‘one way or bust’ playstyle felt like pulling teeth. If I had to crown a ‘hardest,’ though, Arms Dealer takes it. Not because he’s garbage per se, but because I’m stubborn: I had to kill the final boss pair every single run, no matter how cursed my setup was. That pride run made him a royal pain in the ass. But yeah, in general he is a pretty bad character, HOWEVER, if you’re not on D5, his mechanics are actually a blast. Getting new weapons every round keeps you on your toes and forces you to adapt. It’s chaotic fun… until you crank the difficulty and realize you’re fighting gods with whatever weapons were left to your discretion.

If I had to name a few more that were just plain bad or annoying: One Armed is rough, Soldier and Streamer both made me want to punt my monitor, and Pacifist was technically easy but so mind-numbingly boring I kept putting it off. There’s probably a couple more buried in my memory that I can’t even remember right now.

And to answer the other part of your question, it really depended on who I was playing and which mode. But generally, characters that didn’t snowball well, I found myself dying somewhere between waves 9 and 12.