Warcraft 3 Frozen Throne Walkthrough for Alliance:
Chapter 4: The Search for Illidan
Main Quest: The Demon Within
Main Quest: Escort Illidan
If you played the secret level between chapters three and four, you will start this mission with an extra hero, the Pandaren Brewmaster. He will help immenseley here. The primary point of interest, other than the cage is a mercenary camp to the south. Any time a hero dies, have them visit to gain more reinforcements for your fight. When you first start, you can only afford a few of them but they will still be a huge help. Grab what you can, and take all three heroes toward Illidan's cage. The Blood elves will automatically build reinforcements for the fight and send them in waves. The sooner you reach the battle, the better because, as you know by now, heroes can turn the tide of battle quickly.
In the initial fight, use your heroes' abilities somewhat sparingly. Kael should use banish repeatedly to keep a unit out of the fight. Mountain giants are a good choice because their taunt and beefy defense will keep the Blood elves tied up fighting them for a while. They're good to take out last. Use forked lightning and flame breath to hit front-line attackers and pound them fast. Flamestrike can be used but be careful as it can hit the brewmaster. You should get control of the cage on your first push.
Just because you control the cage, you shouldn't stop fighting and escort the cage. It's more important to help the blood elves and Naga hold out the fight. Follow them and fight where they fight. The more distance the cage gains toward your base the better. Heroes resurrect almost immediately when they die, so keep sending the dead to the mercenary camp on their way back to the fight. Thankfully, Maiev is the only enemy hero you'll encounter but the size of the night elves' forces can be daunting. Keep up the hero assaults and get level 6 abilities as soon as they're available. The brewmaster's special is quite nice, as well as Kael's Phoenix. Use them tactfully so that you don't waste the ability. It's useless to summon Kael's phoenix when the Night elves are about to win a skirmish because it will just be brought down quickly.
If the night elves do gain control of the cage once more, you should of course push for getting it back as soon as possible. If a Naga/Blood Elf assault is on the way you might want to go into combat with them so that your heroes aren't overwhelmed. At any rate, this mission is not that difficult to complete because your heroes will swing things in your favor rapidly when used correctly.