There's one Praetorian AV for the Dimensional Warder badge that's harder to get than all the others, and it isn't Anti-Matter like most people think, but Dominatrix. Every Praetorian AV but two can be fought in either incarnate trials or the Apex/Tin Mage TFs, all of which are short and fairly popular content, so it's easy to get teams to run them. The two you never fight are Infernal and Dominatrix; Infernal is still easy to get, his mission is always the first mission Maria Jenkins offers, and you can't outlevel her. However, Dominatrix is only in two missions: the penultimate mission for Maria Jenkins' long story arc, and a standalone mission that Maria Jenkins won't give you until you finish her story arc. So, basically, the only way to defeat Dominatrix for the badge without getting lucky in a PuG is to run the entire story arc, just for her. She's the only Praetorian AV that you can't run a short mission, TF, or Trial to get.
So basically, I would like to see that standalone mission Maria Jenkins has made available to flashback. That would make Dominatrix much easier to defeat.