Well, to be fair, Thanos didn't actually beat Champion in combat....he held his own until he could trick Champion into destroying the planet they were on. He probably wouldn't be able to beat Champion while he had the Power Stone. Thanos really does like getting into a good brawl every now and then though... but he never loses sight of his actual objective. With Champion, all he really wanted was the Power Gem. So, he extorted it out of Champion in exchange for not leaving him floating helplessly in space for eternity.
Thanos' strength and durability are not to be trifled with however. He's tanked hits from Galactus and even managed to knock him down.
Obviously kind of catching Galactus off guard...but still impressive.
Even Galactus is impressed. He has to actually exert himself.