Mangog's power-set has him containing an infinite amount of hate and rage.
The hate of a billion billion beings that were at least of Asgardian Power levels.
Silver Age Mangog appearances stated that his power and strength were without limit.
And, it isn't only the hate and rage of those billion billion beings that feeds him.
Also, violence and hate against him, will power him up, as stated a couple times while Thor hit him during battle.
AND, when he disguised himself as Odin, he was powered up by worship and love.
The only thing that has tripped Mangog up in the past, is that he doesn't seem to totally understand what he is or how he works and certain attacks and strategies have caught him by surprise.
Odin was never even able to HARM Mangog. Silver Age Mangog was never shown to be injured in any way by any characters or weapons.
Odin states that Odin cannot be stopped by anything in the universe and there is no use even trying to fight against him.
There is also a classic scene where Mangog holds Thor like a doll, while he is trying to get the Twilight Sword. Mangog is hardly aware of Thor hitting him with Mjolnir over and over, in the same exact spot, as he simply carries him around.
Was Odin ever able to harm Galactus? Think about that.