I'm going to
Try prove it's sheer power from evidence in the show rather than a Hax ability:
Crowley - "Cant Blame A Girl" S7E1 - YouTube - This scene pretty much proves Power=Hax Resistance, also, Sam and Dean have no idea of how to take him down, even though they know of the Colt, which hurt Lucifer, and other vastly powerful weapons. Castiel also became immune to weapons, Angel blades for instance, they also though the same about Death in season 5.
Crowley says in a later scene : "Bind, enslave Death. You having a laugh". "Lucifer Did it". "That's Lucifer". "A spell's a spell". You really believe you can handle that kind of Horsepower. You're delusional". So yeah, it'd take vast power to bind Death.
Death also laughed off Castiel when he said he'd "Take him out first".
Now Death was bound by a spell that the weaker God Castiel could break, so he might not have the exact level of hax=power, however, specific spells work on specific beings better, so take that with a grain of salt, another likely explaination is that it had fulgurite, which is lightning after striking sand, which they refer to as "an act of god crystalised forever", so it likely is just more effective against Death rather than anything else, as god was the guy who bound him before.
Also, as i said Castiel couldn't smite beings stronger than him, and Reapers haven't Reaped beings with Superior power, and have never reaped a non human/non primordial monster character, in fact, when Angels and Demons die, they just go to the empty, overseen by a Cosmic Entity.
It should also be noted that high level Demons are immune to Angel smiting, Alastair, The 4 Princes of Hell, Cain, Lilith , etc, and Archangels are immune to Angels in general, and this is due to sheer power, as shown when Sam smites Lilith, Castiel smites Raphiel, Raphiel makes Crowley immune to smiting, Jack empowering Castiel allows him to Smite Dagon one of the princess of Hell, which he couldn't do before, and more.
Ok, so inconclusion, Death killing God is
likely sheer power and not hax, he also said he'd do it at the end of time, so the circumstances are unknown, however, it's safe to say he's God Level, power=Durability in the verse, and Death has been shown and implied to be durable in the past.