is there an easy way to do this mission? cause i really hate this one :P does just rushing work, for example?
The best way is split your forcess up into 2 largish groups and rush the Ork strongholds. You should have more troops then the Orks early on in the game and it shouldn't be to difficult. If you try to do the level point by point then every loss you take is in the favor of the orks as they can build bigger and bigger armies as time passes.
Hopefully I have the right map :P
Which race are you playing as? It is a hard mission but I have won it on hardest with several races, although the strategy does depend a bit on which one you are.
Here are some general guidelines: the key is to keep most of you units alive while still acting quickly. Watch out that you don't take too much damage to your vehicles, which don't regenerate and you may not be able to repair. If possible, keep your commander in front to take the brunt of the firepower but don't die, since he (she) will regenerate fastest. Try to entice the enemy away from turrets so you can gun them down with minimal losses. When attacking turrets and buildings, get in fast, making sure your strongest unit (Baneblade, LandRaider, Avatar, etc) is the first to begin taking fire. Anything that begins firing at it will normally keep on attacking only it. Send jump troops to quickly destroy whatever will hurt that main unit the most (missle turrets most often). Move the rest of your troops in as quickly as possible, destroying everthing that does damage before taking out the harmless ones (generators, etc). Try to get badly wounded units out of fire so they are not destroyed, since they often can continue fighting even when nearly dead.