Pooh's type 2 immortality is the only one that is combat applicable. His types 3 and 4 are overtime. Pooh's type 2 allows him to survive being hit by a car and smashed into another car with enough force that it should destroy his ribs and spine, as well as endure beatings that no one could realistically survive.
The Muskoxen has a slight AP advantage. Pooh scales from crushing a human skull with his foot, which is a feat calculated
here. The Muskoxen has its AP value listed on its profile as slightly higher.
Assuming that Pooh has standard equipment, I would give him the edge due to his weapons, pain tolerance, survivability, intelligence, and ability to summon bees. The Muskoxen has an advantage due to its strength and horns, but the gap is small enough that everything else Pooh has should get him the win despite a fairly hard battle.