I guess FFXIII would be pretty tough if solo'd, so that's the hardest one in my opinion. You need a party with well thought out Paradigms to successfully take an enemy down. Some of the Esper battles were solo and/or two person jobs, and they could be pretty tough if you didn't know what you were doing. Also, there's lack of areas to grind in, and Gil is difficult to come by so you can't buy the items you might need.

As for soloing itself... it'd make the game uninteresting for me. I like developing my party, and sharing out magic isn't that much of a big deal - if I know enemies in the next area are weak to ice, then I equip everyone with ice weapons or magic. Summons are different - I give the best ones to my favourite characters/my party, and share out the others however to level up.

If I've 100%'ed a game already, I especially wouldn't have the energy to play again, especially solo. And 100%'ing something is debatable... some people are only happy with their 100% if they finish an FF game with not only all hidden items, characters, bosses and magic discovered and mastered; their inventory has to be stocked with ninety-nine units of every possibly ninety-nine accumulated item.