Tactics for fighting enemies in Iram of the Pillars

As you've gathered, the basic melee combo is pretty weak. Drake will do more damaging combos if you swing at an enemy close to a wall, a drop or an object that Drake can use as a weapon. The trick is getting an enemy close to the wall; thankfully, you have a handy grab move for that: press circle and hold down the direction on the left stick you want Drake to try and fling them.

--WARNING: Spoilers--

I went through the same thought process, but to answer your question, the hardest levels I found were:

• Cruisin' for a Bruisin'
  • In this chapter, my method was to stay in cover about 90% of the time, and shooting only when I needed to. I aimed for headshots most of the time, as when you reach the ballroom of Rameses' ship you'll find one or two Tau Snipers lying around the back. You'll also receive a Tau if you manage to stealth kill the guard nearest you once you enter the ballroom after defeating the heavy wielding the PAK-80. Most of the guards in here have shotguns and grenade launchers, so dispatch these first. Several snipers will appear as well, so don't let them catch you off guard.
    • To defeat the Heavy, use the overturned freight crates as cover. Since you'll only have a Para-9 (and maybe a KAL-7), headshots will work best, as the Heavy will fire at you in rapid bursts that will kill Drake in a matter of seconds. After landing a couple of headshots, the Heavy will throw multiple grenades at you. Use this time as an opportunity to climb up the crate and land a few more headshots. After this, the Heavy will call in backup, so dispatch the grunts as quickly as you can and take one of their KAL-7s. Find cover again, and use the KAL-7 to deliver a stream of bullets to the Heavy's exposed weakpoint. Now that he's defeated, he'll drop the PAK-80, but it's next to useless as Drake will drop the gun as soon as you enter the ballroom.
• Sink or Swim
  • This is a bit of a tough one, as you're constantly on the move for the duration of the level. Ignore the Mag-5 and find guns that have common ammo (Para 9, Arm Micro, KAL 7, Shotgun) and use Run-And-Gun to deal with any guards that appear. Aiming is useless as the boat is always rocking back and forth.
• The Atlantis of the Sands
  • Possibly the hardest Crushing Mode level in the game. Fortunately, even on Crushing, you're still provided with plenty of extra ammo lying around. To defeat the "Djinn", use the M9 to land headshots and then run like hell to the entrance of the big tower next to the elevator, where the SAS-12 is. Pick it up and use it to deal with the djinn quickly and efficiently, albeit you may lose some health.
• The Chateau
  • Simple technique here, just stay in the same cover for the duration of the level and throw grenades at incoming soldiers. Use the AK-47 and the Arm Micro to deal with grunts that get close.