Well Thanos never faced the LT. If he had, he wouldn't have stood a chance. LT is right below the TOAA, which is basically God in the Marvel U. During the Infinity Gauntlet saga, cosmics and others went to LT for him to intervene and put an end to Thanos' madness. The LT didn't think Thanos was doing anything worthy of his judgement, so he allowed Thanos to continue to shape the universe in his image, slaughtering heroes and cosmics along the way. The infinity gems are always powered up, it's just that the gems only work in their perspective universes and everyone doesn't know how to access their full power.
