This is developing in an interesting direction. So far, it looks like Blossom and Kitsune are more or less evenly matched, Proteus doesn't have any particular combat powers to speak of so I'm not too worried about him, Melchior has a general command of protection and telekinesis which means he might be able to take out both Nimble and Piper, and the only reason Invictus isn't dead yet is because he's literally Nigh Invulnerable.
At this point, I think the heroes still have a shot at winning, but, if Fenris stops letting himself be distracted, then that could change very quickly. I'd say that the best move is probably to blow up one or more of the artifacts and withdraw. Gathering Intel is probably the best thing to do now. They went into this unprepared and are now reaping the consequences of that action.
As an aside, I'm assuming that all of the Mythica's powers draw on the Cultural Sphere. As for our Heroes, I think Invictus draws on the Personal Sphere, Blossom draws on both the Cultural and Metatextual Spheres and both Piper and Nimble draw only on the Cultural Sphere. Is that right?