So you wanna summon daemons, huh kid? You want to keep your heresy to a minimum, and work within the fluff? While any army (except the shiniest of the Space Catholics) can work off of Malefic Daemonology, my experience has been in summoning daemons with daemons, or occasionally, with those who aspire to daemonhood. My prime focus since 7th came out has been working the daemon factory. It's a simple machine at it's heart, but like the humble Honda Civic, you can customize to your heart's content. To work properly, you're going to need to summon a minimum of two units of daemons a turn, preferably three; cast cursed earth from a central location, and bring out at least one herald. That's right, I want you to summon 285-400 points of units a turn.
The core list:
4 Heralds of Tzeentch, ML 3, Disc of Tzeentch
17x Pink Horrors
17x Pink Horrors
8x Screamers of Tzeentch
8x Screamers of Tzeentch
1186 Points
Heralds:
These boys run with the screamers. West Coast says that you can't generate the same power out of a unit twice in a phase, so two to a screamer squad. Tzeralds generate powers off of the Divination, Tzeentch and Malefic tables. They come with Flickering Fires. You're rolling Malefic looking for cursed earth and sacrifice. Cursed earth is your main force multiplier, increasing daemon saves and preventing scatter of summoned daemons. Sacrifice makes Sleralds on seeker chariots or Tzeralds to buff your warp charge. Incursion is awesome if your opponent is playing keep-away - you can brind in more screamers. If you get a shite power on your last roll, take summoning. You'll want at least two heralds with summoning: they can jet to the backfield and dump units.
Pink Horrors:
Backfield champions and warp batteries. They come stock with Flickering Fires, which will get cast maybe once a game. Roll their powers last on Malefic. If you haven't gotten cursed earth yet, or only gotten it once, take it; otherwise, swap out for summoning. These are your main summoners: they can easily soak perils that will cripple your two wound Tzeralds.
Screamers:
Your only anti-tank and your primary harassment tool. With cursed earth or jink, you get to ignore 58.3% of wounds with a 4 up save re-rolling 1's. They can mulch light infantry with slash attacks or a charge. HoW and 4 attacks on the charge turn blob squads and small groups of elite infantry into paste. 36" movement a turn with the turbo-boost pushes them wherever you want on the table.
Daemonettes:
They're not on the list, but they're your absolute best choice for summoning. Fleet and a +3" run puts them in your opponents face. They kill everything except Land Raiders and Imperial Knights. You'll want a minimum of 40 daemonettes. It's rare to get more than that on the table, because your opponent will be killing them.
Plague Bearers:
They're not on the list, but your best choice for PtA missions because of T4 and shrouded. Touch of rust can kill vehicles, but don't count on it.
Deployment:
One of the reasons daemon factory lists are so maligned is their incredible ease of play. Between the 12" summoning range and the incredible reach of screamers, your initial deployment is not as vital as with slower armies or those with a small amount of models.
Horrors should go on or near objectives. In ruins, they can go to ground for a 3+ cover save, ignoring 77.7% of incoming wounds. Daemonic instability keeps them from fleeing. These will almost always survive the entire game and give you end-of-game ObSec claim units.
Screamers and Tzeralds should go in front, close to the deployment line. You're going to be aggressive with them, and they can always get away from charge threats.
Against drop pod armies, Grey Knight strike forces or infiltrators, use your high model count to push them into awkward positions. You can easily keep your entire backfield free of invaders, or make a tempting gap for your opponent to foolishly throw a unit into. Place the Tzeralds behind the screamers and bubble-wrap with the horrors.
Against template spam armies like Thunderfire Cannons or IG bombardment, take the time to space your models to the maximum extent and use long lines with the screamers. Don't give your opponent free wounds because you're lazy in deployment! Check to make sure they get one hit with a small blast and three with a large one.
If you have good LoS blocking terrain, try to make your opponent shoot your horrors over your screamers. They are cheap, survivable, and you can easily make more.
Being the Raid Boss:
If you've fought big bads in video games, you'll know what the boss is like. If you could focus on him for just a minute, he'd go down. Unfortunately, mobs keep coming in that will kill you if ignored. The primary focus of the first half of a summoning game is keeping your threat level high. Every summon should spell immediate doom for your opponent, so they are forced to focus on your 'free' units as opposed to your 'free making' units. If they do fire on your heralds and horrors, your adds will punish them with rending charges. You will win if you make slightly more units than your opponent can kill each turn.
First turn:
Keep your finger on the "Make more Daemons" button. It's easy to fall victim to trying to spam FFoT or other Witchfire Tzeentch powers and kill units, which won't work. You start with 18 WC, so lets say you have 22 dice. I like to throw a minimum of 7 dice at summoning for a 77% chance of success, usually throwing 8 dice for an 85% chance. If I have 22 dice, I'd summon daemonettes with 8 dice twice out of both my horrors after casting cursed earth with 6 dice. You can use less dice for cursed earth if your opponent doesn't have a psychic heavy army, but a turn without cursed earth is a terrible thing. Any smart opponent will throw all their dice to deny cursed earth. If you have it twice, I'd try with 3 dice twice.
Against a gunline or static army, the screamers will move forward 12" so their cursed earth bubble covers the new no-man's land. That gets filled with the two squads of daemonettes. In the shooting phase, you curse your dice for a shitty warp storm and run your daemonettes forward. You're looking to weather a turn of shooting with 4++. Try to bait your opponent into shooting your screamers by talking about how awesome they are.
Against an assault army, you're making a wall of daemonettes to either soak the charge or limit your opponent's options. If you won't be facing much shooting, I would throw 8-7-7 to summon three squads of daemonettes. Get in their face; against most assault troops, your opponent will feel ballsy and charge the daemonettes, who will weaken their units with I5 and 2 rending attacks hitting on 3's.
Second Turn:
Keep your finger on the "Make more Daemons" button. Your screamers will get a charge, killing vehicles, or a slash, killing some support infantry. Screamers like living in the backfield, denying your opponent's ability to hold objectives late game and destroying the things that can kill daemonettes. You should be close to initial strength. Don't get disheartened because you lost most of your summoned daemonettes. They were essentially free. Appear disheartened to make your opponent feel like he has momentum. Summon more daemonettes and keep the pressure on. Each turn, you narrow your opponents options as the wall of daemons pushes against them.
Third Turn, Fourth Turn:
Keep your finger on the "Make more Daemons" button. By now the major threats to the daemonettes should be dead thanks to the screamers. Your opponent will look sad. Don't let that get to you; you came here to win. If one squad of screamers has been significantly weakened, make a super-star with the stronger one and hid the weak squad to contest or snipe late game. Make walls with your daemonettes to prevent your opponent moving towards objectives if you can't kill them.
Fifth Turn:
You'll usually have a concession by now. If not, summon to hold objectives. Daemonettes can easily disperse with a run to completely wrap an objective. You opponent's backfield should be in shambles and yours should be strong.
Playing for Objectives:
PtA is the summon factory's hell match. You'll want to push maelstrom missions. This list gives up first blood easily, so you need to StW and deny your opponent linebreaker to win or tie. PtA is the one match-up that you may want to push for Tzeentch powers. Bolt of Change and Infernal Gateway can kill reliably. Every game of PtA I've played has come down to first blood. Play keep-away if possible, or aggressively gun down a weak unit. Remember your screamers can use Witchfire powers and turbo-boost. I pulled a match vs. Grey Knights at the BAO this way by killing a Land Raider for first blood and making a wall of daemonettes that Draigo-and-company couldn't quite get through to deny linebreaker.
The Emperor's Will is another difficult match. Because your opponent can turtle to pull a tie on primary, push for a maelstrom victory and a primary tie.
Your first target on objective-based missions should be your opponent's ObSec units. Summoned daemons, while troops, do not have ObSec because they are not part of a detachment. You'll be looking to contest most objectives and hold one or two. It's very easy to get primary in the scouring, big guns never tire, crusade. It safe to ignore the relic until the fourth turn or so, focusing on destroying the most mobile parts of your opponent's army.