Hmm, that might be a possibility, about the nuke thing, but I heard who was said to be severely injured when the Havok Nuke detonated; seems more likely that he got hit if that's the sace. And I once thought it was too a while back, but actually, Chief shoved some kind of grenade up his chest; but that didn't kill Ur-Didact. I personally think it should remain the same.
Well, the armor locking if hit with enough force does sound a little to similar to the little resistance to melee attacks. But the 2 kilometer fall from Halo 3, which was calc'd at
Wall level, actually causes his armor to lock up. Don't know other instances.
I explained the details about his weakness to melee attacks above, but I guess I can repeat them. I have both the game mechanic details and the scientific details for that weakness. In game, there are two bars, one for health and the other for shields. Most weapons such as bullets usually chip away the shield meter, but don't damage his health until his shield depletes first. Some attacks, especially melee attacks, straight up significantly damage his health as well as his shields. This even happens when the Spartans initially have full energy in their shielding. And they die in 3 punches, normal punches to be specific; Spartan Charge usually kills other Spartans in 1 or 2 hits, and Body Slam oneshots them. And keep in mind, the
9-A ratings technically come from Spartan Charge, meaning their normal punches would be lower, but still in the
9-A range. Now that I got the game mechanic details out of the way, I will explain the scientific details. What basically happens when Spartans get punched by other Spartans is that their suit gets pressed into their body; thus causing the titanium plating to cut their skin and break their bones. It also messes up some of the programs of their suit; kinda like kicking a computer causing the computer to glitch all over the place. So basically, it requires
9-A AP with physical blows to do this in canon, but it does show that Spartan durability is somewhat lower against blunt force trauma than they are against bullets or explosions for example.
As for the electricity based weapons, I noticed the
Energy Sword was considered to be composed of electricity. Also, Spartans have demonstrated being susceptible to
this,
this, and they can get stunned by
this. FanofRPGs and Soldier Blue might also know others.