I play my best mate about once a week. The only army I run is eldar, he has several. He is a a tournament player, and I enjoy the challenge.
He has defeated me with Chaos space marines, grey knights and orks in the last 5 games, using a very similar playstyle in each game.
Through whatever means available, he has most of his army very close to mine by the end of T1. With orks it is a combination of his vehicles and the 'da jump' power. With grey knights, half his army can shunt forward and there is the gate of infinity. With CSM it took him a bit longer, but he had an army of fast things and deep strikers.
Anyhow, he uses that board control really well. I find that I end up cornered, albeit with reserves that come on - but he denies my ability to deep strike really well (I use spiders, war walkers and 20 guardians in the webway). By T5 I have usually put a pretty big dent in his army, but he has been controlling objectives and clocking up points for 5 turns and wins the game.
I find that I struggle to shift his units, who seem able to weather a lot of shuriken fire. I use doom and shuricannons / shuricats, supported by reapers or war walkers with bright lances. I feel we lack a good counter-assault choice, and while our shooting is good, it tends to only take out a single target per turn.
I have experimented with lists, and have a pretty decent collection. Typically I take 2 serpents, a guardian blob, jetbikes for support /scoring and heavy weapons on vypers and war walkers.
Last game I tried dropping the serpents for more small units, which didn't really help.
I have found it useful to take a sacrificial max unit of guardians, who deploy across the edge of my deployment zone. Means that he can't deep strike in my zone, and the stratagem that gives a 4++ is useful.