Okay...point given, Sparda makes Dante leagues stronger.
That's not going to fly, a videogame is going to have special effects. You can say the background that flashes for a few seconds makes Mundus cosmic level, but that's not going to fly as far as proof goes. Especially since his attacks don't do much, his stream of meteors that grounded Sparda Dante could only splash off the pool of lava. Watch the landing scene again, Dante lands without destroying the ground at all, neither do the meteors that obviously badly hurt him.
https://www.youtube.com/watch?v=UUCBTVuWAmE Start watching at 5:40.
Really? Mundus' name? That doesn't mean anything, Gemini Saga's Galaxian Explosion would not be accepted at Galaxy level if it didn't also have, feats, WOG, and databooks backing it up. You are not going to make Mundus universe level using his name as evidence. Another plane of existence? That could very well refer to hell you know.