I've seen a lot of people say that you don't see any good tactics posted on the Internet. Supposedly list building trumps tactics and strategy. Well I have always tended to stick to the following tactics:
-Target selection/ If I am up against an army that is better in assault then I will typically target their faster units first in attempt to take them out before they reach my line. Next I will shoot the slower units that can unleash hell once they get stuck... If they are in a transport then you've got to at least take out their ride and make them foot slog. Also I always favor high odds that have the best chance of paying off as opposed to high risks that require a lot of luck.
I believe in the concept of Focused Fire. Once you have selected a target keep shooting it until it's dead. Too many times I've seen people spread their shooting around. This let's remnant units win games. How many times have I seen a couple of genestealers pop five terminators? Believe when I say it happens. Or when a single Marine runs on top of an objective. I remember once watching a gladiator match at Adepticon. My friend was running L&tD... He charged a Blood Angels Sanguinary High Priest joined with a squad of assault Marines. The Blood Angels assault squad was destroyed and the priest broke but did not fall back far enough to be able to regroup. My friend decided the next turn to ignore the priest and assaulted another unit getting stuck in. The Sanguinary High Priest then regrouped and lead three small las/plas squads into a counter charge... Preferred Enemy + furious charge... They destroyed a large block of mutants which won the game for the BA player. Now if my friend had charged the priest again it would have gone the other way for sure. So you have to remained focused at all times, especially when you are winning a close game.
I see shooting as broken up into three phases: long range, mid range and short range. Against assault armies you are waiting for them to move inside your midrange arc... The short ranged units move up and suddenly you can blast them with everything. It's good to have a balance between the three ranges. You don't want too much of any one type.
Assaults win games and this is most especially where you want to focus on high odds. If I have an IC joined to a unit I'll put all of his attacks on the enemy unit and ignore the enemy IC... generally your IC will hit first, hit on 3+, ignore armor and has a lot of attacks. Kill the enemy squad and their IC will break and possibly you can overrun it too. If you focus on the enemy IC it's harder to hit and typically has a better save so you are wasting attacks that could have killed more. You also want to charge multiple units so you can tie them up. Coordinating your charges is very important.
• Mission Objectives
How often do I see people say one army can beat another solely based upon the two lists? It just does not work that way. If you focus on objectives rather than trying to table your opponents you'll be in a much better position to win late in the game. By playing smart you can neutralize your opponent's best units and force them to not use them as best intended. Killpoints are all about tabling but when it comes to controlling objectives that is a completely different ball of wax.