Is it possible that Thanos defeated the Living Tribunal, because of experiance?
Thanos: The End was the first time (that we know of) that the LT had to contend with an enemy of such power, so while LT may have mountains of power, he has very little experiance in havening to use it in battle.
Thanos on the other hand, has tons of experiance with using power. He's a seasoned warrior and skilled tactician. He's very familiar with the offensive and defensive use of power in combat.