I don't know if there is a way to write a universal work on tactica in a single entry. However, I do know what works for me, and here are a few of the thoughts that helped me win with an army that isn’t supposed to.
TO KNOW AN ARMY: I really work hard to learn the codex. This one is pretty obvious... but sometimes there’s value in looking past the obvious. The Dark Angels Command Squad can be loaded with melta-bombs for example. Place them in a Land Raider, arm them with close combat weapons, and they're versatile assault troops. I think they might be better at assault than my regular Assault Squad choice... I haven't tested that yet. To me, everything is theoretical until it's been tested in the field, which is why it’s particularly important not to become tied to a single theory. The best example I can think of is my Dread, if I kept running it around like a tank, it never would have made me money. Of course I accept that some units are just useless, but that’s all the more reason to know which ones exactly.
PERMUTATIONS: The second step consists of agonizing over my choices. This takes up much of my brain. I want to be able to contend with all threats and remain true to my preferred theme. Like many people, I'm sure, I run through many iterations hourly. Build the list and rebuild it until I like what I see. I don't have a secret, I just know when it looks good to me.
But that's all just academic. Everyone builds his army differently, and I would not want anyone to change what works for him (or her). Games day is where I like to think my 20 watt bulb flickers to life... (like those dim, flickering lights in horror movies not quite revealing places the hero shouldn't go.)
THE UNCARVED BLOCK: I don't believe in starting with a rote battle plan. I try to know my potential, have an idea of what I want to do, but don't set myself on a specific path just yet. Be the uncarved block. I like to flank. Given the choice, I will drive up both table sides and keep you hemmed up in the middle. Failing that, I enjoy the flying wedge, work up one flank, move up that side of the table with more heavy hitters, try to get to the side of cover so that the enemy cannot hide from the wrath of the Emperor's First Legion. So… what if I my opponent has strong anchors on each side? I cannot employ either tactic. Perhaps I should let one side or the other advance into my guns instead... Or fall back from that side and let him meet nothing but air... I find it best to see what's going on and adjust my tactics from there. If the board is more or less even and I get to set up first, then I try for an even deployment, cover as much as I can, adjust as necessary on turn one.
MUTUAL APPRECIATION: When I deploy I try to keep every unit supported by something else. My Devastators have the Land Raider filled with Termies on hand. It won't keep a Deep Striking unit from mauling those guys, but the Termies can then move in on the next turn and minimize damage instead of having enemy units run rampant in the back field. No unit is an island, not even Terminators. Unless you're using a unit as purposeful sacrifice, everything must be covered by something else.
Also, it doesn't hurt to have an anti-armor weapon (power fist, melta-bomb, lascannon, Holy Hand-grenade of Antioch, etc.) in every unit. There seems to be a lot of armor out there.
SEIZE THE INITIATIVE: No, I'm not referring to dice here. If your opponent wants to funnel you somewhere, fall back instead. If he's laying out a strong defense to keep you away from his tanks, wade on in (if you think you can take it). At my FLGS, the popular idea seems to be that we lay out our armies and then meet in the middle. Accordingly, I don't go to the middle to meet. In refusing to play his game, you force him to play yours. If you get to choose where and how the fire fight goes down, you are more likely to win.
During that last tournament, a Space Wolf player showed up with neither tanks nor transports, with the exception of two Dreadnoughts, his entire army was on foot. This already gave him the upper hand. It was contrary to expectation. I asked him after; he said he did surprisingly well. Which brings me to my next point... Don't listen to the interwebs. Sure, there is good advice out there, but don't let someone's "uber' list become your list. If you’ve read it, there's a good chance your opponent has too. They've already considered how it might be beat. Show up with a list of your own devising, and they don't have weeks to ponder it before going to war. Plus, as that Space Wolf player showed, the universally accepted idea that 5e must be armor centric is wrong.
TABLE EDGE: Unless there is an objective to protect, I don't really care about my edge. You can have it. Most people have been conditioned by sports to protect the "home" side, not to let the other side get control of it. In 40k, who cares? Let them have it. Move your troops out to more defensible positions. If you're lucky, they left a few units to hold their starting area and you can cherry pick them at your leisure.
AAA: The last thing I consider is advice from Gen. George S. Patton. It goes with my previous point about seizing the initiative. Attack, attack, attack. The first turn or two you might jockey for good position, but when you're where you want to be, hit him with everything you have and don't stop. Sure, those few tacticals you have left from that squad have a single melta-gun against a line of tanks. But they can't make money of they don't engage. True, they may die on the way to optimal range, but they may not. Gentleman's Ones happen all the time.
[B. Smoove: Thank you Rob for a great series of posts! It's fantastic to see that the Dark Angels have not been cast to obscurity, and today's tactica has been particularly thought provoking (I confess that I have the tendency to guard my board edge like it matters, despite playing a scout-heavy army. shameful, I know). Thanks again.
See you all tomorrow as we return to the regularly scheduled program of Hailed vs. Hated.
Oh, one more thing, images used without permission but without intent to violate copyright or other forms of intellectual property, etc.]