I've got an old Dragon Magazine that has an article on tactics and this is one of the big recommendations: Target your fire.
The "1.a" of this is that you should start with the squishiest foe, even if they aren't the biggest damage-dealer. As lowkey13 pointed out, this is a math problem, and it looks a bit like a compound interest formula. If you want to pay down your debt, start with the easiest to pay off loan and continue to snowball the money onto other loans when it's done. There are obvious exceptions, like when one of the enemy can one-hit anyone but is still pretty tough while the others are kobolds with BB guns; but you should still consider the odds around overwhelming forces (which is kinda what kobolds do).
Note: even though I called this a "math problem", this is a case where math models reality, not a gamist solution. This is a fairly real-world tactic and it still makes sense to remove soft-targets that could harry you while you're laying siege to a bunker.
So, the answer to the first question is, and always has been, "yes".
The answer to the second is a return question: "How tactical do you want your game combat?"
For me, there's a line that crosses suspension of disbelief, but I also take into account NPC intelligence and temperament such that wolves are likely to "wolf pack" but ogres probably won't. Further, that dark knight might also be willing to engage in 1-on-1 combat for glory.