Doom is beatable but it's difficult because what makes Doom great is his ability to predict what you will do and prepare for it. He is a master manipulator. This is a man, who in the first Secret Wars, while all the villains were trying to win the Beyonder's prize, and the heroes were just trying to get home safely, Doom was plotting how he could steal the Beyonder's power. And he did, by manipulating Klaw, taking out Galactus and using that power to steal the Beyonders. You can never count out Doom, and my personal opinion is that among all the Marvel villains he is second only to Thanos and even that may be open to debate. I would love to see a story pitting the two of them against each other. Epic may be an understatement for that battle! As far as the group of heroes you chose, Phoenix would be a worthy battle and could possibly beat him, as could Dr. Strange. Martian Manhunter's vulnerability to fire against Doom? No chance for Mr. J'onzz. Deadpool is a big wildcard but I could see Doom manipulating him like he has Klaw, Molecule Man and many others. And Squirrel Girl...lol who knows?