A good tactical tip is always say on mission, you would be surprised in how many players think every mission should involve wiping your opponents force off the board, and how that can be their downfall...
Always ask yourself these questions during a game, and always give them an answer:
If your playing an objective game, how many objectives can you realistically get? I say realistically because there maybe an objective that your opponent has swamped with objective grabbers.
What units within that mission can take objectives? is this Big Guns Never Tire? Scouring? or even The Relic? know whats in your army that can take the objectives of the mission.
How will you push your opponents squads off the objective? say there's an objective on center the table and your opponent has it, how will you combat it? will you try to wipe it clean off the board? will you demoralize the squad to the point where it has to continually take moral checks every time you shoot it?
imo its always best to not kill the whole squad off unless its fearless, why? because you don't want to focus fire at one thing as your giving the rest of your opponents army space to breathe, only do so if you feel it absolutely necessary to do so in front of a dangerous unit, that you feel requires your attention.
Once you have an objective how will you protect it? its good and all to have a squad on that objective, but that squad will die if there is nothing to assist it. Will you screen that squad with your wraiths? will you provide a dangerous fire support with a large threat range for shooting? will you cast a spell on you objective grabbing unit to make it more resilient and therefore survive more?
In terms of kill point games, think of these questions and answer them whilst fighting:
What units are good at what? are the wraiths good at shooting? are the Annihilation Barges good in
CC? what roles represent your force at excelling?
What do you want to put to the blade? what units do you want to kill, that you think will die quickest?
What units do you think should combat units your opponents have? do you think the wrath squad would excel against the bike squad? or do you think it would be better to fight the 5 man marines squad.
Similarly know where your units are placed? do you think your wraith squad is better suited to charging the 5 man unit that is 10" away? or is it better at charging the bike squad 4" away?
Will your unit survive the initial combat? if you move your Barge towards a marine squad out in the open to shoot at it? do you think the marine squad will survive and if not will your Barge survive being shot by a devastator squad with 4 lascannons? if your wraiths charge the biker squad, do you think it will survive the first turn of combat if they get in
CC with them? if not where do you think they would be better suited elsewhere?
Also look at the secondary objectives of the mission, as they usually become the game decider of the game your participating in, as I have learned from past experience..
Another thing to note is don't get too sucked in with killing one unit that your opponent has and become a Moby dick on the table, I have seen a lot of games where a player has a chance of winning what appears to be close game then throwing it all away because said player has the odd fetish for killing that one unit like Moby Dick hunting the whale, heck I've done it multiple times on the table with my Helldrakes accidentally... It always bites you on the backside in the end...
Always have this in mind as it makes your mind highly flexible for majority of games your will have, and it can be quite hard to do this in
40k trust me as
40k has deep imbalance, but in majority of games you will learn what you did right and what you did wrong and how you could improve for the next time..
Just my opinion, hope this helps