I finally did the "in utter darkness" campaign map on brutal, after 3hrs of trying.
I tried alsorts and never got past 2000 kills (the target is 2500). Then, boom - 3260 kills.
In case anyone is interested:
* research improved shields and armour right from the start (essential!)
* warp in dark templars during the first 10 mins or so (before the void rays arrive). They're pretty much invulnerable because very few overseers are sent early on
* during that time, just move your forces from entrance to entrance depending on where the attacks are coming (they mainly come from just one entrance at a time early on)
* once the void rays are available, spam them like crazy. I found 4 starports to be enough. I had 24 void rays towards the end.
* you'll be limited by vespene gas, so use spare minerals to build point defence canons. I found a tight ring around the archive building worked much better than trying to wall-off the entrances, as it lets your forces cluster.
* when the second round of nydus worms come, you (probably) won't be able to hold the lower ground any more (attacks increase in intensity massively after that). So move up and wall off the entrances with point defences as long as you can...
I tried alsorts before this... massed collossi (no chance - far too delicate), massed carriers (no good - run out of fighters and are too slow), and massed dark templars with fighter cover (great until overseers show up, then they all die!).