Just gonna lay this out, to get an idea of how much more powerful Cosmos is, her original form required the combined forces of Discord, the Elements of Harmony wielded by Princess Celestia (IDW) and Princess Luna, Queen Novo's orb, King Aspen, and the king of Abyssinia to seal her away, with Discord noting that it would require all 6 of them to banish her, meaning that any combination using any less than all of them would fail. The EoH are powerful enough to defeat Discord, and the statement that Novo's pearl and the magic of both King Aspen and the king of Abyssinia would be needed along with his chaos magic and the EoH to defeat her proves that she is easily more than twice as powerful as him. In her fused state, she has all of her own power combined with all alicorn Twilight Sparkle (Pony World), Zecora, and Big McIntosh. Considering the fact that the combined magic of the 4 princesses was enough to match Tirek after he grew powerful enough to drain Discord and than added Discord's power to his own, basically making his power double that of Discord's, Cosmos in her fused state must be well over 4 times as powerful as Discord, having all of her base power (which by itself should be greater than all alicorn magic Twilight or second to last key Tirek) plus all the alicorn magic, Big Mac, and Zecora. So, yeah, fused Cosmos should be the most powerful being in the entire MLP franchise (G4 or otherwise) bar none. Also, while she never displayed many of the feats Discord did, it is likely that she can replicate more than a few of them, considering Powerscaling and the fact that he was afraid of her in her original form. We don't see her use as much hax as him, but there is nothing saying she can't.
I consider this a stomp in her favor, though I am inclined to listen if others want to argue that Discord could possibly remove her stars and depower her by utilizing certain strategies considering he is bloodlusted and she's not.
He’s bloodlusted so he won’t mess around here. And he canonically knows Cosmos’s weakness so he’d know what to go after. He quite easily handled her while still mostly messing around once he got over his fear of her.
Well, she IS well over 4 times as powerful as him, so she DOES have more than enough power to defeat him in a single stroke. Problem is, while Discord is far less powerful than her (well less than half as powerful as her normal state and well less than one fourth as powerful as her fused state), her fused state has that weakness in her stars. Ironically, if she fought him in her far les powerful normal state, he probably wouldn't stand a chance. I would say this is a "who strikes first" scenario. I have a tendency to vote for the more powerful in those scenarios, though it is worth noting that Discord is bloodlusted here while Cosmos is not.
Cosmos has much more AP and durability and far more powerful reality warping. It should be noted that while she did not demonstrate several of the powers Discord has, she likely could if she wanted to. Her profile notes "likely far more (powerscaling from Discord)"
Her being listed as slower than him was an oversight that I corrected on her profile. The facts that Discord is bloodlusted while she is not and her fused state has those stars as a weakness are what gives him a decent advantage. If he was not bloodlusted or she was too or if this were her base form, he likely wouldn't stand a chance.