This seems to be where the difficulty steps up, at least for Insane. You must destroy Webway Gates all over the map and reach some sort of ancient Chaos dig site. You'll have to struggle, but be patient and you'll win.
You start with no base or Servitors, but you only need to take out one group of Eldar to get those. Tie up their infantry with melee units and shoot with Tactical Marines. Your Dreadnought must take on the Wraithlord, and I suggest you help with the Force Commander. You should win with minimal or no casualties. Move a bit north and you get to base-building.
Now it gets a bit nasty. Eldar will attack your base from the forest southwest and from the river west of your north SP. You'll want something like a commander, a heavy bolter squad and a vehicle (melee Dreadnought or Predator with big lascannon and two bolters) to both locations. It's tough at first because you don't really have what you need - especially if they bring vehicles. You may have to beat at some early tanks with your commanders because they could come before you have access to missile launchers, not to mention vehicles. On the other hand, they don't have anti-vehicle weaponry either, so once you get a walker/tank they can't really hurt it. If you kept your Dreadnought fully repaired, it can do a number on the early opposition here.
Additionally, you must help the Imperial Guard troops holding back Ork attacks up north. If you don't constantly keep an eye on them, they'll be gone soon, and you know who's going to be the one holding back Orks then. There will be frequent assaults by a combination of a Big Mek, a Killa Kan and a Looted Leman Russ Tank supported by infantry. So you want a minimum of 1 missile squad, 1 heavy bolter squad and 1-2 vehicles (Dread or Pred equipped like before) guarding the Guard. Put your units in front of the Guard units, as yours are more durable and if you lose a tank you can rebuild it - if you lose a Leman Russ, you're not getting another. When the attack comes, have your anti-tank weapons target the Killa Kan first and the Looted Tank second, while the others kill infantry (do not let the Killa Kan get into melee!). When the attack is over, quickly repair all vehicles before they come again. They attack often, you have no way to stop it, and it will go on for the whole mission; it's very tiresome. Try to get a nicely arranged defensive setup into place, so it can take care of this threat with minimal micromanagement on your part.
So when you have a defense that holds up at all these locations, you can start taking out Webway Gates. Scout with Skull Probes and start wherever you like (they are all over the place). You don't need a big force - 1 vehicle with 1-2 squads is usually sufficient. The Eldar production bases are in the forest next to your base, in the other forest way up north, and in front of the huge ramp thingy northwest. Taking out their stuff is actually very easy, the Orks will likely trouble you more.
They also have a weird secret base only reachable with jump packs but forget about that one. However, if you want to complete the secondary objective (destroy all Eldar bases) you must annihilate it too. This is actually easily accomplished by clearing out the lower area and parking four Whirlwinds flush against the bottom of the impassible slope, then ordering them to perform a continuous blind bombardment ("attack ground") until you've killed everything. To check on the progress of the destruction and adjust your bombardment, use an Assault Squad to jump in for a quick peek, and then immediately jump out before you lose any men; keep an Apothecary standing by to heal them up while they wait for their jump packs to recharge to full, before hopping in for another peek. Once there's just a handful of resistance left inside, the Assault Squads can jump in and finish it off themselves. It will take some time, but since you're bombarding them with total impunity, this will be no problem. Keep your Whirlwinds handy, because once you're done here, they have more work to do on the final Eldar base (the main mission objective).
It's the final Eldar base that will turn your hair gray. When you push up the big ramp where there's Slag Deposit, stop to build a secondary base here before moving on, it'll save you time as you're about to lose some troops. Then push on and fortify around the SP right before the Eldar base. Upgraded LP, turrets, Tactical Squads, a tank or two - what you can fit there. Note that the approach is rather tight, so you if you want to minimize casualties, you have to start pulling a weakened unit back early, taking into account a possible traffic jam between your units. When you reach their base, they'll start making crazy counterattacks with hordes of infantry and Brightlance Platforms (and the Orks are still attacking every few minutes too). The Brightlance Platforms will turn your vehicles into Swiss cheese and if you move too close, they will bomb you with Fire Prisms from inside their base. Remember that Eldar Weapon Platforms are heavy infantry, not vehicles, so shoot them with plasma. Though they have lower sustained damage output, their long range means well-positioned Scouts with Sniper Rifles also help against Platforms. One flamer squad demoralizing their infantry hordes will help you a lot.
Whenever you beat back an attack, try to push inside their base and kill something. Your first priority would be the Fire Prisms. Maybe take out one at first and another one on the next try. Throw something in to distract and try to missile/lascannon them from distance. Your two HQ units (with maximum upgraded toughness from the Sacred Relic) actually work well up front as fire magnets, on account of their highly resistant Commander armor type; just have an Apothecary waiting outside to heal back to full after each attack. Two Predator tanks fully upgraded with lascannons plus a couple of small missile squads (4-man or 5-man with max missile launchers) will be able to move in, blast a Fire Prism into wreckage (or at least take a big chunk out of it), and get out again without blocking each other in the narrow chokepoint. Repair/heal up, rinse and repeat. As always, use Battle Cry to buff the missile launcher Dev squads. It gets a lot easier after you get those Prisms and the central Webway Gate down. There's also a ledge on the right overlooking over the base, where you can be a nuisance from; it's hard to reach any good targets from there but you could try Whirlwinds or jumping Assault Marines in to melta bomb the base; you can use the same force that you put together to wreck the secret Eldar mountain base earlier. Once the major threats have been pounded into dust, move up en masse and mop up the area.
When you crack their defense and reach the dig site you will win the mission. The next mission is Sacrifice.