Shhhh.... are you trying to get UDK nerfed?
slap a heal reduction on him and he falls like the rest.
You may not have a heal reduction champ though. At least not a consistent one, many UDK are built with high resist so landing it can be an issue anyway. Teams are also built around protecting him (and by extension, the rest of the team) as well. Remember, once everyone else is dead, the first hit appears to be completely mitigated (this is probably a bug), only the second hit does damage, so you need multi hitters or AE hitters to do any sort of damage. If you dont have those alive you are not going to kill him. Plus any healing you do to your team heals him as well, that complicates the whole killing part.
That is basically the core of the problem. The issue damaging him once the rest of the team is dead (he is VERY tanky at this point), plus the protection provided by other team members prior to that.
At high levels the approach is to kill him first and hope the rest of the enemy team does not wreck you while doing that, while enemy teams are designed to prevent you killing him first while at he same time wrecking your team. None of this is healthy for the game IMO.