I'm currently in Hob's Fall Cave, trying to recover the Helmet of Winterhold, as a level 6 Kahjiit on Master difficulty.

I reached the part towards the end where you open a gate by pulling a chain. Going past that point, I eventually come across a group of Apprentice Necromancers who are incredibly tough. I run as fast as I can back along the tunnel, through the gate and pull the chain.

The mages will basically run into the gate, casting spells at me. I tried using destruction magic to defeat them but it is too weak, so instead I went with the idea of running in front of the gate, slashing a few times with my sword, then running behind an obstacle while I recover. I found that this is also incredibly slow, but I did notice that my one-handed skill seemed to rise rather well, so I thought it would be a good idea to keep doing this until my one-handed skill gets high enough to be able to kill the mages quickly before they can full-heal.

I did this for about half an hour, and shortly after my character reached level 9 my XBox froze. Resetting the game, I tried again, making sure to save the game after every level-up. Regardless of what I do, once that battle has lasted a while the whole XBox will hard freeze. There was even a time where I levelled up, and the console froze when I came out of the level-up menu (I was standing in front of the gate and used the level-up as a quick HP refill so I didn't need to run back).

Is anyone else having these problems?

I have patch 1.7 installed, and the game is installed to the HDD, but I never actually tried that particular cave before so I can't say for sure if this has been caused by the patch or if it's always been there.

I did Hob's with no problems but at level 20 something. Do you have Dawnguard installed? 1.7 so far has not caused me any problems but I don't have Dawnguard either. I also don't use autosave features. It may be the patch. When 1.6 first came down it did similar wierd stuff to my game. I had to start all over.

No Dawnguard, just basic Skyrim installed with the 1.7 patch.

I don't know if it's the necromancers that are themselves glitched - I've killed a few on the way to this point without too much difficulty, but these ones are seemingly immune to everything I try and I can only do slight damage to them even if I use my sword. Once I get them down to about 10% health, one of them will always cast a full-heal type spell to completely heal him before I can kill him. As soon as I get my sword out they will immediately cast a protection spell, as soon as I 'think' about casting a spell they will put up a ward and block the damage. It's literally gotten to the point where I have them stuck behind a gate, allowing me to duck behind an obstacle and heal every few seconds or so.

I can't really tell what's causing the hard freezes. It looks like the mages themselves are glitched, but either way I'm forced into a very long battle that could also be the culprit, rarely seen anywhere else since fights don't usually take that long to finish.

It sounds as if they are confused. When an NPC is critical for some later quest they will do something very similar to what you are describing. Try leaving the cave and coming back in.
Ex: I tried to wipe out the thieves guild way back when. (didn't know I couldn't) We battled for what seemed an eternity. My one-handed skill went to 100 and still all 4 of them were reviving and rejoining the battle. I finally ran out of the place to take a break. When I came back in with my two-handed weapon, they acted as if nothing had ever happened. Needless to say I had to reload a previous save and lost all my new skill level.
I went to Glennmoral Cove to check it out. No one is there. I know this will not stand true as that is a location for a Companion quest later. Might be the same thing happening with Hob's at such an early level.