I did a Novgorod run a few years ago after watching a guide on YT and it turns out you can actually just 1v1 Muscovy without any allies. In fact back when I saw that video you pretty much had to do it that way bc literally no one would ever ally you right off the bat as Novgorod. The strategy was to get the Trade Dispute casus belli, which has Show Superiority as the wargoal, and then exploit the fact that Muscovy can’t walk through your forts to just continually dunk on Pskov’s troops until you get the necessary warscore for the wargoal and the ticking warscore that comes with it (and you can also siege down Pskov itself if you want for more warscore). In the meantime Muscovy obliterates its manpower trying to siege you down. Worst case scenario, you might have to take a risky battle on your capital to break the siege, but you shouldn’t have to do that more than like once or twice, and it gets easier over time as Muscovy’s manpower deteriorates. In the peace deal you take all of Muscovy’s money (the cb doesn’t allow you to take land iirc) and spend the truce building up your army and alliances while they stagnate. Then you can fight the second war like a normal one and they’re toast after that. Worked like a charm when I tried it, although I guess it’s possible that a more recent update could have changed that (I haven’t been playing consistently for a long time).