I've succeeded with all quests on solo using the wizard and priest. After playing through delve with the same characters, they seem to be the most powerful combination. I like all of the characters, but those two have the ability to generate a huge number of success tokens which allows them more of everything, and the priests aid text helps to keep the wound cost of the wizard's abilities down. As far as I'm concerned, using them in solo Is easy mode, but I am also known to be a damnably lucky die roller.
I need to experiment with some other combos to be sure about this, my conclusions are mainly based on mental calculations. P/W will not ensure a victory on quest 3, but they have the best opportunity to complete it.
I'm looking forward to going through the main quest a few more times with different characters to see how it plays out, but I enjoyed it. I felt like I always had pressure on me to make good decisions, and just a couple of bad rolls can put you in a nasty spot as the peril track advances.