No you can't do most seals Solo: no to Flawless, Enlightened, Shadow, Blacksmith, Rivensbane, Curse breaker at minimum. Aside from past and current cheeses Outbreak is extremely tough to do solo, and Divinity can not be done. Whisper obviously can but for average players is extremely tough as it has an absence of scaling for one player and it could and should have. Same with Dungeons - they're not comparable - what is easy- medium for a preset fireteam to do is many, many times harder solo : there is no scaling. As I said Raids, Quests, many Seals, Dungeons, Trials and Heroic-Master levels are a significant part of development offering some key loot out of the reaches Solo-play. That could be better worked out but it's likely it's a trade-off, a deliberate Win/lose on the calculation that it benefits revenue that way.