Who killed him first is a simple fact. Like others already pointed out it was ensidia. So if your question was indeed meant literally then it was a very easy one to answer which you could have looked up on the main page.
If you meant who defeated the encounter first and deserves credit for it, it (not quite as obviously but still kinda) was Paragon. Not only did they defeat the whole encounter and not just parts of it but they also did this on first try. Which imho is much more worth than who did it first anyway. Especially on non hard encounters the "who killed first credit" usually goes to the semi competent guild who logs in first. So not really a measure of skill.
About the ensidia exploit. They used their normal rotations and that happened to bug the encounter without really being able to tell why. With having limited tries on that no one with a sane mind would have canceled the encounter, thus wasting valuable tries and started debugging blizzards game like some demanded of them. But what they should have done is report that bug (since at least that something irregular was going on was obvious). So they made a mistake. But imho Blizzard made two. First not finding that bug before and second the choice of punishment. They should have reset the encounter and taken the loot/achievement, thus giving them a second shot at the unbugged version. And maybe on top of that the 72 ban for not reporting the bug.
I am all for a hard line against exploiters. But it is not like they knowingly and actively exploited the encounter like the yogg+0 incident mentioned before. And the impact on this punishment is quite severe. It keeps one of the top guilds from participating on the really important 25hc kill run. So i think Blizzard went over the top here. Delaying 25hc progression on one of the top guilds for 1 week is a stupid move to punish not reporting a bug. So if ensidia now happens to kill lichking25 hc within 1 week after first kill we will have wild discussions about who deserved credit again .