I've soloed every story mission from Vanilla, TDB and HoW. I've also soloed several in TTK but not all since my friends have been super hyped to play with me for some reason (Sunbreaker maybe?). It really isn't that bad. Some parts are tricky but not a big deal if you are a decent PvE player. [spoiler]I even soloed the part on the Regicide mission where you have to fight the 2 mini bosses (The colossus guy and ogre) at the same time and completed it without dying. I'm decent at PvE, not amazing, and all I had to do was play smart and keep moving between cover points.[/spoiler]