In a recent thread over on The Warhammer Forum I offered some advice to a Space Marine general playing against Chaos Daemons. Having used my Blood Angels against Gary's Chaos Daemons in six very tough games I thought I had something to add. I also thought I'd reproduce my thoughts here so they are preserved for posterity.
I've played half a dozen games against a Daemon army with my Blood Angels and I've done best when keeping virtually all my army off the table in reserve. It negates the deployment advantage Daemons normally have because you get to see where they set up which means you can target isolated squads. It also means you can refuse a flank and effectively put some of his units out of the game.
Take the second turn when you get the opportunity so he has more of his units on the table and you get the vital last turn to grab objectives.
In one memorable game I set up one combat squad with a missile launcher on the table to try and tempt the Daemons toward my lines then hit them with my firepower and combat ability from reserve. Of course, my army had Death Company with jump packs and units in Razorbacks and Rhinos, but regular Space Marines could still pack a decent punch if the daemons are within 18" of your board edge when your reserves come on.
If the Daemons don't go for the bait straight away then your superior shooting should force his hand later.
Just be careful to compete for the objectives as the game progresses.
A unit of 10 marines should be dropping at least 3-4 Bloodletters with one round of shooting, 5-6 if you can get the flamer template on them. Concentrate on shooting each daemon unit until it is completely dead, then move on to the next.
If you can, get the Dreads into close combat with his Bloodletters and other lesser daemons. Most of them can't even hurt the Dreadnought so they'll be stuck there just getting killed. That should leave your Tac squads more room for manoeuvre. Fearlessness works against the Daemons here and you can munch through them surprisingly quickly.
The alternative to using reserves is to castle your forces. This suits a more static, shootier Space Marine force.
Deploy everything on the table at once if possible, all in one small area at the corner edge of your deployment zone. That makes it dangerous for your opponent to deep strike within 12" of either board edge. If you don't leave big gaps between your units he won't be able to land right next to you and he'll be taking a big risk by deep striking with 12" of you. That means he'll probably come in between around 18" away from your lines in a big clump of units.
You should split the Tactical squads into combat squads and put the missile launcher half on the outer edge of your lines. They are going to die horribly in the service of the Emperor. Make sure you keep enough space between each unit so that the Daemons can't assault two units at once.
Everything else will be behind. When the Daemons assault and kill the front line you move all your squads up and barbecue them with your flamers. They should be quite nicely clumped following the close combat. Alternatively you can throw your own units into assault, especially the Dreads and Termies. Sure you're pretty much guaranteed to lose 15 Tac marines but then you get to choose the assaults the round after.
This tactic will only work if you have better shooting than the enemy and again you should be careful to keep an eye on the turn so you have enough time to claim objectives.