The opposite to taking this indirect approach is, naturally, to take a straightforward approach that has one obvious objective.
What's interesting aboot this point, is that while it's generally a decent idea in game, it's less so in listbuilding. For the most part specialization in your units is a good decision. You don't want to spend points on a Power Fist in your Devastator squad, for example. If the unit is for shooting, pay only for what they need to shoot. Even more specifically, don't put two H Bolters and 2 Lascannons in the same
Dev squad (Combat Squads aside). Not only should a squad be specialized at shooting, it should be specialized at shooting the big stuff or the small stuff.
Redbeard makes a lot of other good points, but what really stands oot is that you need to be thinking of what the objectives are, and how you're going to win. Don't just be haphazardly killing off his models, and trying to keep yours alive. Know which of his models matter to the objective, and which of yours don't.
Fire Warriors may not seem like much of a threat, but if they're the only models he has that can control objectives, they're a priority.
Conversely, your squad of Terminators might be very expensive, and the core of your list, but if they can't hold an objective, you can afford to take risks with them, and can even throw them away late in the game if they're not near any objectives.
Anecdote: The other day, I played a guy with my Tyranids. He had a pretty rough day with the dice and mission choice. Despite all that, when we rolled to see if there would be a 6th turn, we were in a draw. There was a sixth turn and I tabled him, having lost maybe 175 points of my 1750 point list. As bad as his luck was, and as lopsided as the kill count was, he was keeping himself in the game, and ultimately had a 1 in 3 chance to pull a draw on the roll for 6th turn.
Conversely, I once lost a tournament game in which my Blood Angels rampaged over the other guy's Ultramarines, killed probably 1250 of his 1750, and lost 500 themselves. But all his surviving units were on objectives, and he pulled a win. I didn't even notice until the last turn that I was losing the whole time.
I shot him with my terms, assaulted and killed it right out.
Even if the big bug seems to do well, never forget the cost. For example, I managed to run my Hive Tyrant into a squad of Chaos Terminators. He ripped them up pretty good, before the ones with Fists managed to finish him off. Ultimately he probably killed 3-4. And guess what? At 30 points each, that's still less than the 166 the Tyrant cost. To make matters worse, that was a
KP mission, and while he got the
KP for my Tyrant, one lone Terminator went off and hid behind cover for the rest of the game, and I never got that
KP. So, basically, I gave up my Hive Tyrant for nothing.
So, again, always know what you're playing for. In a
KP mission, you're not playing to "kill some of his Terminators" you're playing to kill all of them, or not to even bother.
I would suggest that you keep some mental notes (or even physical ones) during your games.
This is a fantastic, if somewhat impractical idea.
One thing I think would be a major a benefit is to take some notes on decisions that pained you at the time, and their ootcome. "Charged 6x Terminator (2x PFist) with Tyrant. Expect to wipe the unit." Play it oot. "Tyrant died, one PFist lived." Later you can even run the numbers with Mathhammer and see if you were right in your expected result or not.
What's maybe a bit more practical, is to take a digital photo of the table after each player turn, or better yet, after each player phase. It might not have all the details you need, but if you look at the shots soon after the game, you'll probably be able to recall what happene with each phase. You can even post battle reports, which can be fun.
Another tip: Get together with a friend, some terrain and some models. Take turn setting stuff up, and then guessing ranges. Measure the range while your friend is looking away, then have him guess. In charge range? Within 12? Within 18? Etc. etc. This is a hugely critical skill, especially in 5th.
Take the Dakkafex. With an 18" range, these guys can really kill, but you have to get close. If you elect to try to shoot, you can't run, and vice versa. So, if you measure range, and it's 19", you could have run, gotten to a little better position, and then shot next turn. By measuring and coming up short, not only do you lose that move, but the other guy now knows exactly where you are, and he may opt to move back 6" to save himself, etc. etc.