I'm on 'Tip Of The Spear' and all is going well, I've taken out the first AA gun, destroyed the Wraith around the corner and gone over the bridge. All good so far. I get to the mining facility and use the DMR to take out as many grunts and jackels as I can from a distance and make my way down to the actual building. After battling my way through and killing the Zealot(which seemed much easier than I expected) I realised I haven't seen Kat since the Wraith. Is this normal or have I broken the game? Most importantly, will I still be able to finish the level? It's taken me  hours to get this far and I don't want to start again!

I can't remember clearly about the Kat part, sorry. I just wanted to post to agree with you about how easy the Zealot was. I haven't played Legendary yet though, just Heroic.

I'm on legendary and when he appeared as I came round the corner I just kept shooting him from a distance with my needle rifle. He never fired back or moved more than a few feet.

Just wait until the last level.....

The last level (if you mean Lone Wolf) is the easiest in any Halo game. And the harder the difficulty, the easier it gets. But it still doesnt wreck the game, it fits in nicely.

Job done. I cleared that level and am now in space. Panic over.

I mean the level before the credits happen

Ok yeah, that one is pretty tough. got very stuck right near the end of it. And that was only on Heroic. 

I had the same problem with Kat on that level. Odd. 

And legendary isn't too bad on a whole. Only two parts I got stuck on are the "last level" already discussed and the part instantly after the space combat with entering the Covenant ship in zero G. I beat it legendary on my first play through in two 5-6 hour sittings and I'm by no means a Halo expert.

I will admit though that the part just before the credits was irritating!

You can almost skip all the parts in most missions by the way. That means you can do it in Legendary too you know...! I can't imagine how easy it's going to be with 4 other players in matchmaking (only played with 2 other friends so far and it was easy like hell).

Like priteshmodi said, these parts are annoying. The last part on the "last level" was because there was no good weapon ammo left anywhere, so I had to kill the last few remaining Brutes with plasma pistols. The other part before you get on the cannon is easy though.