Not sure if I can pinpoint to a "hardest" part of the solo run, because I feel like the most annoying thing about solo runs in general is the time it takes to do the run - this took me an hour and ten to twenty minutes, compared to sub thirty or twenty runs I'd do with 4 people.
But there are moments that certainly stand out. For my runs, I've used Crystal Deer Lance Charge trick. The problem is if heavy stander activates against your Lance Charge and you don't use deer hide in time, the Succubus Fiends shoot you and your deer down, forcing you to crisis escape. Another thing is that you can't use your deer to stun the mobs, because you need the deer to hide, and it won't be able to hide if the deer itself were in combat (the stun counts), so either you gotta use shadow cloak right after the crisis escape, or use other knock away pets (flamemares) then summon the deer and run your ass off. There was also a moment where I had all my crystal deers cancelled out due to phantom roar, and had to deal with rest of the mobs by using trans+demi+fh+ms.
Another troublesome moment was the flash bang part. I was trying to unequip bag to get the flash bangs on the floor, but I didn't do it in time, which meant that I had to spend another 20 minutes or so trying to kill the wraiths. Not particularly hard, but rather annoying.
If anything, I'd say boss was the smoothest part for me. I did get the part where I have to hit her 30 times twice, but I opted not to since it wasn't a big deal for me. For others though, I'd say be careful in boss. You want to stay far from wraith as far as possible because getting hit by the roar hear means almost absolute failure.