As Illidan you want to go in (sometimes twice or more), do tons of dmg and go out alive. My Skill build will help you to do this. My skillrotation is the following:

1. Go in with Q (if possible on assasins like jaina, kael...)
2. Hit as much enemies with W as possible
3. Autohits are very important for Illidan, cause they heal him and reduce his cooldowns. So you want to hit your enemies!!
4. Use your E (with shield) and "First Aid" if you get focused (and trust me. You will get always focused)
5. If you drop under 30% max. HP use your Heroic and jump right into the enemies (try to involve as much enemies as possible to gain more max HP from your Heroic) and you will have almost a second life to terror them again!
Many Illidans use their heroic way too early. Dont engage with this skill. Use it as late as possible.
6. If you drop low a second time (or you can finish an enemie, but your skills are on cd) use "blood for blood". Dont try to use it on tanks (even its more worth on them). You will waste to much time with running to them or sth else. Just use it on your target, you are hitting anyways.

Sorry for my not perfect english, but my native language is german :) if you want to see my full Guide in a Video and other stuff about how to climb the ladder, check out the following Link:

https://www.youtube.com/watch?v=ooostzqIJr8