Everybody seems to have this false idea in their head that Superman is just a planetary-level being and he has limits, just like every-other fictional character. However, that simply isn't true. Only an omnipotent can beat Superman and here is why:
Superman is similiar to a solar cell. Superman's body takes in sunlight and converts it to power that he can use. The more sunlight Superman's body absorbs, the more powerful Superman will become. However, what most people don't seem to understand is that there are no limits as to how much solar energy Superman can absorb. The more solar energy Superman absorbs, the more powerful he becomes and there is no limit to how powerful he can become. That's how Superman became Super prime one million. Infact, in the Justice League Series: Our Worlds At War, after Superman spent 15 minutes inside the Sun, Superman was powerful enough to move planets without any effort. Superman's power depends completely on the Sun. This means that the longer Superman stays inside the Sun, the more powerful he becomes. It's simple logic. This means Superman could beat The Living Tribunal. Think about it. Since Superman can become as powerful as he wants, Superman could absorb the power of every single Sun in the Universe, and then imagine how powerful Superman would become. Of course it would take an extremely long time for Superman to stay inside every single Sun in the Universe until it dies out, but if he does, it's obvious that Superman would become powerful enough to beat The Living Tribunal. Not to mention all the other Suns in the multiverse that Superman could absorb. So you see, Superman's power is potentially limitless!