Some people will balk at me calling this power gaming, but strategies to take down enemies by what the system encourages are a form of power gaming. As your GM ramps up the difficulty in the dungeons and encounters you face, you're going to want to know this too. So grab your notebooks, folks, today we're talking about the best way to take down combat encounters.
What Does Your Game Favor
Before we can talk about the best way to take down an encounter, we need to talk about what the system does to provide one or the other. How your system of choice handles combat is going to in large part decide the best way to go about things. Fortunately, for most systems, there is only one big thing to consider and that is how wounds are handled. Namely, ask yourself this question:
As a character (or monster) takes damage, does it become worse at things or not?
For example, in D&D 5th Edition my Paladin is just as good at hitting, and not being hit, at 1 hit point as she is at full hit points. However, my L5R Mirumoto Bushi is penalized on attack rolls when injured. Even worse, my fighter in Shadows of Esteren is worse at both attacking and defending when injured.
This means that the more hurt my Esteren and L5R character is, the worse they are at combat and the lower the threat they are. On the other hand, my D&D character is still the same threat no matter how hurt she is.
Why does this matter? Because how damage affects PCs is also true for NPCs, and odds are in a combat you're going to have a lot of NPCs to fight.
Damage Doesn't Impact Effectiveness
If damage doesn't impact effectiveness than your best strategy is to reduce the total number of attacks coming against your group as fast as possible. This can mean focused fire, but doesn't necessarily. Yes, ideally if all the PCs focus on him they might be able to take out the big threat in one round, but if you can't take that threat out you have a lot of attacks coming back at you. Consider instead what happens f you take out all the small fry? Yeah, the big guy gets his attack(s) off, but less attacks over all are coming in. What your group is capable of (more on this later) will determine the best way, but remember this: your goal is to kill, not to wound.
In games like D&D it is worth sacrificing some damage (i.e. putting a D8 arrow into something with only 4hp left) in order to get the creature/NPC off the board against you. A dead/out creature doesn't make attack rolls, and an attack that isn't made can't hurt you. It definitely can't crit against you.
Damage Reduces Attacking
If damage reduces attacking, it becomes more worth it to spread around the damage. Why? Well, what is more scary. Five attacks against your Difficulty 12 armor class with a D20, or 6 attacks where each attack gets -5? In the 5 attacks there is a 45% chance of you getting hit on each roll. With the 6 penalized attacks yes there is a sixth attempt, but the odds of you getting hit are only 1/5 (20%) instead of almost 1/2. In other words, with 5 attacks you should get hit 2-3 times on average. With 6 attacks at -5 each you should get hit once, but aren't likely to get more than that.
Damage Reduces Everything
If Damage reduces everything you should default to what you do when it reduces attacking. However, in this case it can also be worth having the weak combat characters (or non-combat characters) finish off the injured targets. Why? Because with lower defense they're easier to hit, and it removes the threat completely while still leaving most of the group wounded.
Anyone who can ignore the wound penalties for an attack should obviously not be treated as if wounds impact them. Also, like I said above, you really want to check what your characters can do as well. For example, using a fireball to do 6d6 damage across the room - knowing it will only maim but not kill anyone - is much more viable when someone can follow up with another AoE ability (like Sleep).
In general, Area of Effect can change the game depending on how it is used. But the main point here is to be smart about how you tackle encounters. Especially when the GM is going for making them as mechanically challenging as problem.