It's not that much harder to be honest.
Still wrecking the game on a solo POTD monk ^^
Edit:
OK, the increased difficulty definitely shows, actually.
You get many more misses, many less crits, take much more damage.
At level 20 even on a solo character it's still very doable, but you do have to be much more careful.
Should it even be possible to complete the game on POTD with a solo character?