I'm on the 7th level and so far I've not really found any problems. It usually takes about an hour a level and yes you will die a lot but not because it's hard per say, mostly because you'll make stupid mistakes or the computer will get a lucky grenade throw.
Tips for the 5th level will be shoot the Elite Zealot from a distance with a DRM/Needle Rifle, he wont run away if you do. I found that out by accident just shooting him thinking he was a regular Elite. Keep your rocket Warthog alive, it wont be hard if you drive and let Kat shoot. When you come to the bridge bit where you can't take your Warthog any further just bombard the building with tons and tons of rockets until all the bad guys on both floors are dead.
There's a Revenant after that I think, just get in that and the rest of the mission will be a breeze, it takes 17 or so Revenant bullets to kill a Hunter.
Also at the end highjack a Banshee, fly it to the top of the Spire, land it and get the Elites attention then run away, the Elite will get in the Banshee and you can easily highjack it again to take him down easily.