Spellduckwrong here. Something that I have actively noticed in several games now, both ones I have played in and those I have observed is that sometimes a player will lose a unit because they didn't force the opponent to make a decision. It sounds almost counter-intuitive that you would actually want to give an opponent options, but please hear me out.
To elaborate on this lack of choice thing, I mean, that the player will position a unit (usually a scary one, though not always) in such a way that it is a no-brainer for the opponent to pour their offensive resources into blowing it off the table. The reason it is a no-brainer is because the opponent didn't have to decide how to prioritize targets because in this situation there was only one. An example of this would be, say, a flying monstrous creature burns straight at the opponent's line on Turn 1 far ahead of the rest of its army. And therefore, in the resulting opponent turn, they really don't have anything better to shoot at because nothing is really in range. Especially, for their short range massed infantry fire. Their only choice here is to fire or not fire. Unless they are trying to throw the match, they will choose to fire.
My main point here is that when we don't present choices to an opponent, we will generally fail to gain any benefit from the opponent laying into one of our units. An easy solution to this is to approach the enemy with two "attack groups". Essentially, two small forces that present equivalent tactical utility for us. Both groups need to be strong enough that we are still going to have a good chance of accomplishing our missions as long as either survives. What these groups will be will composed of will really depend on your list and what you intend to accomplish. You mainly just want to avoid having the opponent kill off enough of your forward units that when the remnants of those forces hit, they fail their mission and are easily wiped out in a following turn.
Timing is also very important for attacking your enemy and not removing choice from their battle plan. If you have Big Nasty Unit A (B.N.U.) hitting the opponent's line on turn 1, Big Nasty Unit B hitting on Turn 2, and Big Nasty Unit C hitting on Turn 3, then you have given the opponent an easy choice or no choice but to shoot B.N.U. A the first turn. In subsequent turns, if the opponent has enough firepower to handle your B.N.U.'s one at a time, your tactical situation hasn't really improved and you have merely let the opponent grind up your units piecemeal pitting many of their points versus relatively fewer of your points. How could we improve this? Either make it so at least wo of these units arrive at the same time. Either two pull up turn 1 in the same spot, which cuts the opponent's available firepower in half because they have to split between two targets or ignore one of them. The other option would be to make two units hit on turn 2. Even if this means having one of the units move slower than it would need to. By waiting, we are taking away the opportunity for the opponent to utilize much of its close angled mass fire because we didn't feed a unit into its range in the previous turn.
Overall, don't send a lone unit off to soak up an entire army's worth attacks. Time your advance so that multiple EQUAL threats hit the opponent's lines in tandem. A Rhino with 5 Marines and a Plasma Gun is not an equivalent threat to 10 man Terminator squad. The Rhino can be ignored fairly safely compared to the Terminators but maybe five Rhinos with 25 Marines are just as scary. I guess a good rule of thumb would be to make two groups where you would be happy if either of them was still standing when the next turn's carnage switch is going to get flipped to "ON".
Another aspect of making the opponent choose is whether to move to shoot you or do nothing with a unit. For a basic example, say there are five Bolter Marines sitting on some terrain who, because of range, can only shoot your nearby unit of five squishy troops. Well, if you leave your troops in range of the Marines, they will sit there and fire on them because the opponent doesn't have to make a choice. It is shoot with the Marines at their only possible target or do nothing. If you move your troops either out of range, out of line of sight, or move in a second unit of infantry, now the opponent must make a choice. Move the Marines to go after your troops, pick one of the two targets it has now to shoot, or possibly fall back because now they are outnumbered. You see, once you introduced a scenario where the opponent had to make a decision, there are so many opportunities for them to make the wrong one either by miscalculation or poor rolling for a random element. Either way, the situation is better than just letting the Marines pump Bolters into your troops without a downside.
Always remember to make your opponent choose, don't make things easy for them.
So what units have you witnessed getting blown off the table because they ran ahead and took the brunt of the opponent's attack or just got otherwise isolated?