How did the Beyonders manage to kill the Living Tribunal?
3 of the Beyonders triple-teamed The Living Tribunal just to kill him. It didn't say how long the battle lasted, but it showed the 3 of them blasting him together. That was that! They came, they saw, they conquered!
Because they're abstract kryptonite.
Plot. It's always plot. Things get ret-conned in and out of existence to serve the current story line. So, the Living Tribunal needs to die? And be killed by the Beyonders? Ok! Beyonders are now 1000000000% more powerful than they originally were! That stuff just happens. Although if the Beyonders are in any way linked to THE Beyonder, then that would also make a little more sense as to how they're so powerful, since The Beyonder is really powerful. Galactus was bascially a gnat to him...
The Living Tribunal's power is with regards to controlling all the multiple realities of the Multiverse.
Not sure of the exact methods, but it seems that being extra-dimensional and outside of the multiverse, they have been able to manipulate events on a much higher level, by sending energy from their plane to ours, creating many copies of Molecule man, with the intent of using them as multiverse bombs. It appears that the incursions being created during secret wars was due to the beyonders defeating all the celestial beings in our multiverse.
The Beyonders managed to kill the Living Tribunal because the LT wculd not judge against them. The reason for this is that the Beyonders don't exist inside the multiverse, therefor not under the Living Tribunal's judgement.