I've returned to 40k after a few years out.
I play one opponent mostly, and I encourage him to bring his tournament lists. It has been a hard journey of defeats, but craftworlds are an unforgiving army.
I thought to keep a list of the lessons I've learned, and I may as well so it here, where others might benefit, and also contribute.
I'm on a limited budget so play with my admittedly large, but dated army, so I don't have access to all the tools of like.
1. Deployment. The 9" bubble for deep strike and easy many objective missions works means that table control is important. Where I have done well with what though, I have had to relinquish tabke control and fight out of a small pocket.
Orks are an example of an army that moves quickly -deploying on the front edge of my zone has allowed a first turn charge. Providing only hard targets such as alaitoc wave serpents is important in case you go second (I think I have gone first once in the last 10 games).
2. Following on from this, counter assault is important. I have used an autarch skyrunner and wraithlords, but in my experience, it is rare that I won't want to assault.
3. Concentrate fire. In most games, I have over-estimated my own firepower, and left targets behind that should have been killed (meganobz take a lot to move).
4. Guardians are not cultists, and shouldn't be sacrificed. A full unit of guardians can be buffed well between CP and psychic powers but still costs close enough to 200 points. I have made the mistake of using them as a speed bump. Min Avengers squads and storm guardians with flamers are better options being cheaper and better at Overwatch.
I try to play what where they are kind of a moving Castle, redeploying each turn. Well if like to. What ends up happening is I get surrounded and overrun. I'm still looking for the intended that can deal a lot of hurt. So far:
20 guardians on a doomed target
Farseer with smite and executioner
Dark reapers, but even with fire and fade I find them hard to protect.
Fire prisms with the linked for Stratagem, noting that this is expensive both in points and in CP.
Occasionally, war Walkers with bright lances have been amazing but they are inconsistent.
I had another game last night. It was close, as I pegged back some points late game.
1. Watch out for deep strike all game. I left a pocket just large enough for some meganobz to 'da jump' in front of my war Walkers. With a
2. Fliers. OMG, they are amazing for Eldar. In the surface it is the -2 to hit for being a flier and alaitoc. But, they can't be assaulted, so they can be used to block the space your opponent needs to assault your squishy stuff.
3. Wave serpents. Having run then with 3 shuricannons, I have found them a little lacking in damage output.
Running them with star cannons and twin shuricats I found much better. Range on the star cannons makes it easier to concentrate fire. Three difference between S4 and S6 on the shurikens is not as important as it once was (you really want to roll 6s to wound for the AP bonus anyway).
4. Pheonix stone is a remarkably good relic. I often lose my farseer skyrunner Warlord. Pheonix gem combined with fly gives him a chance to get away. The spirit spear / sword is not as terrible either.