One thing that is certain about boss fights is you should always pack Healing Wards and the heart that instantly heals the hero for 300 hit points (I forgot the name of that item) in Malfurion's Pouch in order to beat any boss you'll face with.
You must always make your Golem engage the enemy hero and buff him with a Fire Shield to make the enemy hero take more damage over time. Golem's Stun ability is very useful against almost any enemy unit.
Now, while fighting against Illidan, save your ultimate spell for nagas, so when both Illidan and nagas attack you, you can hit them all with the ultimate, dealing them 500 hit points instantly. Make sure you get surrounded by the enemy units or just stand in the middle of them, so that the spell can damage every enemy unit around you. Naga Siren units will constantly cast Frost Shield, so I recommend you kill them first WITHOUT engaging any other unit to have some easier time dealing with other units before Sirens cast Frost Shield.
Always look for runes of healing/mana, as Illidan constantly spams with Mana Burn ability, making the hero run out of mana pretty quick, affecting the hero's hit points at the same time. Not only that, but Illidan would occasionally take the runes instead of you (or will he?), so you have to take them before him.
Since you're playing on Hard, I'm sure you acquired some legendary items throughout the previous chapters, so your mana would regenerate faster, making your hero more fit to cast Firewave. So, a good combination of spells include: spawn a Golem, cast him a Fire Shield, than spam Illidan with Firewave until his naga reinforcements come when you'll want to use your ultimate. I don't think about two Naga Myrmidons guarding the imprisoned Highborne, but the other naga units (Naga Siren, Couatl, Snap Dragon and Mur'gul Reaver units) which will eventually come to aid Illidan. The best time to use the ultimate is at the last enemy wave at the top of the map when the most Illidan's naga units attacks you. Also, always make a good use of Fireware to cover as many naga units in a line as possible. It's a great spell, capable of dealing a huge amount of damage to large groups of enemy units. Keep an eye on your Golem's Fire Shield and keep casting it when it's out.
When you're running low of health, put some Healing Wards from Malfurion's Pouch. You have to learn to quickly switch the items from the inventory to Pouch, use them when you must and put them back to Pouch exchanging them with the item(s) you previously changed. Since the enemy AI is only focused on your hero, your Healing Ward will be ignored, making your hero restore the health in a quick time.
Hope I helped