Tactics involves knowing the capabilities of your units and using them correctly. These suggestions should help you improve your tactics, whether you are a beginner or a little more advanced. I would also recommend reading The Art of War (Sun Tzu, many translations available), Infantry Attacks (Erwin Rommel), and all the General Tactics articles at Portent.net. If you want more information on Attacking Time, you might also check out the Wood Elf Archery Army article by Stephen Lorimor, also in the Portent tactics section.
Area of Effect: If it is a Heavy Weapon unit, imagine a circle around it with a radius equal to the weapon range. On the other hand, if it is a close combat unit, the circle’s radius is equal to the movement and charge range. This circle determines how much the unit can influence the game. The larger the circle, the earlier you can engage an enemy unit or support a friendly unit. Keep in mind that circumstances may change the circle, however. That Lascannon team with a 48” range circle will have almost no effect if you park them in the middle of a heavy forest, and the fast close combat unit may be limited by difficult or impassable terrain.
Concentration of Force: There are very few units that can entirely wipe out a unit in one turn, whether by shooting or assaulting. In a unit, the last things to go are generally the Heavy and Special weapons and the close combat monsters, so just killing half of a unit doesn’t make it half as effective. Generally, you want to concentrate the power of several units on a single target, to make sure it is dead. You can prepare for this by having the Area of Effect of your units overlap. That way, when something comes under the guns of Unit A, Units B and C can also fire on it, and Unit D can assault it as well.
Attacking Time: Most games last 6 turns. This means that you have 6 shooting phases and up to 12 assault phases to do your damage. You want to maximize your time while minimizing that of the opponent. Area of Effect is a good way to maximize your time, since longer range weapons or higher mobility assault units can start killing sooner. On the other hand, you want to minimize the time of the opponent. For example, if you are playing against an army that you don’t want to assault you, you’ll have to slow them down. If they reach you in turn 2, they’ll have 9-10 assault phases to do the damage. Each turn that you can delay them will cost them two assault phases of damage to you. The most powerful unit in the galaxy can’t kill your entire army in 1-2 phases. The way to limit time is to limit the Area of effect that your opponent’s units have. Set up terrain to channel vehicles and large assault units. Use difficult terrain to slow them down. Shoot the transports and make them walk. Use terrain and dead vehicles to block their Line of Sight. Use picket units placed ahead of your forces to slow them down. Picket units should be difficult for them to get through, if they are to be of value. This can be due to high Toughness or Armor Save (Wraithlord, maybe Terminators, Dreadnaughts, etc.) or due to large numbers (squads of 20 Guardians or several squads of Guard, Gretchin…) as long as it will take them more than a turn or two to go through the picket. In the meantime, you may concentrate on the rest of their army, or you can move further away, attacking their time even further.
Don’t meet Strength with Strength. If your opponent is strong in assault, don’t try to assault him, use a different way of taking him on. If you try to play your opponent’s game, he will probably win due to experience, and even if you win, it will likely be a Pyrrhic victory. On the other hand, if you use a method that attacks his weakness, you stand a better chance of coming out nearly unscathed. Even in smaller things, don’t get in a shoot out with a Devastator squad, assault them. Don’t charge an assault squad, shoot them. If you put your best assault squad against your opponent’s best assault squad, even the winner will probably no longer be a viable unit, but if you shoot the heck out of them, you will be more likely to survive and shoot the next turn. The first time I faced Necrons with my Dark Eldar, I had been playing a shooty Dark Eldar force. I read up on the Necrons, however, and decided that trading shots with them would tear us both to ribbons, so I made an assault army and had a field day.
First things first. You can’t do everything at once, so you need to decide what to do first. Figure out which units in the opposing force do what and which ones are the most dangerous to you, based on their power and Area of Effect. If you are a shooty army, for example, enemy transports will be fairly high on your list, probably followed by highly mobile assault units. A Ravener 17” away is closer than a Genestealer that is 13” away, due to their different areas of effect.
Out of Sight, out of mind. Cover is your best friend. Even if you have heavy armor, there are always weapons that can turn it to slag, but cover will give you some protection. In addition, if you use the terrain right, some or all of those weapons may not be able to fire at you. If your unit can’t see their unit, they won’t be able to see you either. You’ll still have to watch out for units that can move and fire, but you may be able to eliminate them through firepower. You can also use your vehicles and troops to screen. Troops with heavy armor will block a lot of firepower that might have otherwise killed off your important squads. For that matter, troops with light armor can do that too, but they’ll quickly lose their effectiveness. Sometimes that sacrifice is worth it. Vehicles also block LOS, but this works both ways. Your squad behind a vehicle cannot be seen, but it also cannot see, so vehicle screens tend to be more useful for assault elements or Guess Weapons. You can also use firepower to help protect your units. If you maneuver a unit so that terrain and vehicles block LOS to it from all but one enemy unit, then fire on that unit until it is destroyed, it will be very hard for your opponent to destroy your unit.