He and his minions are very difficult to solo, and there's no sigil. I just spent half an hour asking around Cenote: "Hey, you fought Wanadi yet?" "He's right over there. Big myth boss with two fire zombies." "No? What about you?" *Jump domains* "Hey, you fought Wanadi yet?"
I think the team-up feature has really spoiled us. :-)
And yes, there are probably other boss fights that ought to have the sigil added as well.
I agree he should have a team up sigil, but I also disagree because it took m forever to beat him but when I finally did it felt so good, same with clemente moraga. He is very hard so I understand why you want a team up sigil. But I say he should stay as a solo boss.