If you were a time traveller, fighting heroes and villains at their strongest would be plain dangerous or suicide, instead you should travel to a certain moment in history where those characters are at their lowest and kill them there!
Easy concept, the idea is to show a moment in a character's history where he is at his weakest or at a low point, and try to find the weakest moment for different characters.
I suppose that Superman being depowered every other issue should help.
Most comics have stipulations on time travel that prevent such things. Rip Hunter, for example, has to maintain the timeline by only intervening where necessary, not whenever he pleases.
Kang wouldn't cos he would want to fight heroes at their best to try and prove his superiority. Any old time traveller can kill someone whose at their lowest ebb. A villain does it when they are at their peak Which causes their inevitable downfall.
But there are characters that even at their weakest could kick ass.
I should've probably said something about attacking them after becoming heroes at least, I just wanted to point the historical "low moments" of characters in their history
Technically, the Doctor (Authority) has done this. He sent Midnighter back in time to kill Jenny Quantum's evil twin, because the MNer's senses were the only ones that could distinguish between the two at the time. And it was amusingly effortless on The D's part.
Kinda makes subsequent stories a little harder to swallow, when he could be one-shotting villains with a cradle punch, but doesn't. Writers usually do a good job of finding a plot device to keep him from doing such things, but not always. Which is why the time travelling "rules" I mentioned earlier are generally easier to work with as a writer.