This works against infected and melee humans. Dodge behind them and aim for their legs to sweep them to the floor while possibly crippling them. Enemies are more vulnerable on the ground and will ragdoll as you hit them. Just be careful of humans with body armor/helmets, you will deal no damage to them AND they will not ragdoll!

Infected will DODGE YOUR ATTACKS if you don't attack them from behind after a dodge. A weapon with longer reach may help but to guarantee hits, dodge behind them, then hit them.

Chain crippling a horde will make a possible threat become laughable as they limp towards you.

Example:

https://gfycat.com/welllitlikelygreatargus

This also works against infected and melee humans. With a hunting knife, you can dodge behind them and crouch to assassinate them, even if they have seen you! You can also knock them down with a leg sweep and as they are getting up, sprint over behind them, crouch, and Assassinate them!

So the flow is like this:

Infected tries to grab you->Dodge behind them->Crouch->Assassinate

As an added bonus, this will also level up your Stealth and Hand to Hand skills incredibly fast!

Example:

https://gfycat.com/energeticimmaterialkitty

NOTE:

Infected will NOT attack you while you are Assassinating another infected, they will try to grab you but they can't. This means you can chain Assassinate by immediately trying to dodge after killing the first one. This will NOT work on human enemies since they will hit you while you are Assassinating another.

Usually, when you aim with a firearm or bow, the reticle will shrink down until it is locked down and red, ensuring a guaranteed hit. However, you can get a guaranteed shot if your target is in your face. Even if the reticle is not fully locked on to a target, you are guaranteed a shot if you are at pointblank range. This means that you can get easy guaranteed head shots if infected or humans are charging right at you.

Example:
https://gfycat.com/secretnastyicefish

While I'm not sure what the modifier is, sneak attacking does extra damage. A great way to abuse this in order to level a combat skill (Archery, Firearms, Hand to Hand, Stealth) is to CHOKE HOLD an infected, sneak, and hit them.

To Choke Hold, don't have a hunting knife equipped and sneak behind a target. Equipping a hunting knife changes it to Assassinate, which in this situation, we don't want.

For easy Choke Holds, refer to tip 3 but without the hunting knife.

NOTE: Leveling combat skills seems like it is based off of DAMAGE and not HITS.
Therefore having books that increase your combat skills will also increase your damage and rate of leveling!

When you panic, you may want to just spam dodges and run away from danger. Unfortunately, that actually drains your fatigue meter even faster. When you're in combat and not locked on a target, all movement turns into sprinting which means moving far distances will destroy your stamina.

Locking on a target and strafing side to side lets you conserve stamina and may occasionally dodge infected setting you up to counter. This is without actually dodging and just strafing side to side.

Example:

https://gfycat.com/soupyblaringbandicoot