User blog comment:B214/Question and Answers 6/@comment-185.112.105.215-20161212173441/@comment-10330822-20161213101733

Yeah, Ultimate-Class/Satan-Class is a safe assumption for the group. Tannin in particular is stated to have power that rivals the Four Great Satans, meaning he is most likely Satan-Class. Tiamat was likewise stated to be Satan-Class and the strongest Great Dragon King by the Familiar Master.

As for Vritra, Migardsormr, and Yu-Long, that's a little more up in the air. Vritra is sort of hax because of his OP, cheat abilities that are supposed to make up for the fact he isn't as strong as the other Great Dragon Kings.

Now, Outrage mode is a different story, because it made Fafnir strong enough to kill Rizevim, who was a Super Devil. Among the the Four Great Satans, only Sirzechs and Ajuka are Super Devils, however, they are considered much more powerful than Rizevim. So, since Rizevim was killed by Fafnir in Outrage Mode, it would probably stand to reason Tiamat and Tannin could probably kill a Satan-Class, at least in Outrage Mode. This would mean that they could at least kill Serafall and Falbium, especially as a group of Great Dragon Kings.

Sirzechs and Ajuka are the wild cards here, since the are members of the Top 10, I don't know if Outrage would be enough to kill them, even if it does made the user Super Devil-Class. Likewise, Sirzechs and Ajuka are supposed to be far strong than normal Satan-Class. Meaning that without Outrage, logically none of the Dragon Kings could defeat them by themselves. As for a group, that is hard to say. If they all used Outrage to become Super Devil or Satan-Class, they might kill Sirzechs and Ajuka, but that is assuming their combined might is more powerful than Sirzechs and Ajuka's might.

Since the Super Devil-Class is the highest class of devil, the ceiling for its power could be vast. In fact, its possible the different between Sirzechs/Ajuka and Rizevim was like the difference between an Satan-Class Devil and a High-Class Devil. Rizevim could not even have been in their league and just been stronger than the original Satans.