How do I fight multiple enemies effectively?

Fighting one tough opponent is no problem. Dodge, attack, dodge, etc. The "automatic aiming" ensures I will always hit when in range. But how are you supposed to fight multiple opponents, surrounding you, striking you from behind?

In other games like Darksiders or Assassin's Creed, you can just press a direction and the protagonist will lash out in that direction. This way you can always attack the enemy who poses the greatest threat.

But in The Witcher 2, you can only ever attack the opponent currently selected, and even when performing a slow attack, wielding the long sword in a wide arc, only the selected opponent will be hit. Sure you can turn the camera to have the game select another foe, but it's extremely "sticky". (I'm playing with Mouse+Keyboard. I also have an Xbox 360 gamepad)

Currently, I run around in circles, wait for the monsters to line up, hit the first one a couple of times, and then run like hell. It's neither fun nor does it feel very heroic.

For instance: How did you fight off the Scoia'Tel that attack while Triss casts her force field (at the beginning)? I don't think I killed a single one of those elves. I was just running/dodging around Triss until we arrived at the village.

I've written up some tips on the Witcher 2 combat on the Gaming.SE blog, here are some tips out of that for dealing with multiple enemies.

Most of the time you will be fighting multiple enemies at the same time. What makes these fights difficult is that you receive doubled damage when hit from behind and that you are staggered for a moment when you’re hit and unable to do anything. The most important thing by far is to never get surrounded! Once you’re surrounded you will take double damage from the enemies behind you and the enemy attacks will stagger you and preventing you from escaping quickly.

You should always stay on the side of the battle if possible, not in the midst of it. Use a light attack from the distance to close in on an enemy, preferable one a bit isolated from the group, and roll away before the other enemies can surround you. Hit and run tactics are very effective in this game.

To fight multiple enemies at the same time you should always open the combat with a sign from a safe distance. Using Axii to turn one enemy to your side is a very good opener, it means you have one less enemy to fight for a short time, and this enemy will distract other enemies and allow you to attack those.

Another very good option is to set one or two Yrden traps to immobilize enemies. Once you immobilized one, you can attack a different enemy without being in danger from the immobilized one.

Once you’ve used your signs and decide to attack with your sword, use the Quen shield on yourself. This means you don’t get vigor regeneration, but it will protect you from damage. Even more important it will prevent you from getting staggered while attacking an enemy, which makes it far easier to roll away after your attack. This sign probably makes the biggest difference in combat, always shield yourself with Quen when you’re attacking.

Using Aard from the sidelines and then leaping in with a light attack is also an alternative. If you get lucky the Aard sign stunned the enemy and you’ll perform a finishing move and instantly kill the enemy.

If all this isn’t enough, you need to be prepared better for the fight. Drink some potions before the fight, you have some Swallow potions from the beginning and you should use them for the difficult fights. Set some traps if possible and throw your bombs or daggers (if you have the required talent). You’re meant to use all these consumables in this game, don’t hoard too much.

My answer to big group of enemies is Aard Level 3, plus the Energy Flow talent that increases chance of critical effects.

If you invest points into the mage path, you can get up to 4 Vigor without difficultly, allowing you to mass spam Aard. At level 3, it has a decent range and AoE, and as mentioned in other answers, give you a chance to stun other monsters. I have taken down big foes easily by killing them after stunning with an Aard.

The biggest, baddest crowd control skill, however, is the Heliotrope sign. To charge it up effectively, make sure you have either Igni or Aard at level 3, because damaging lots of enemies at one go is the best way to charge your adrenaline.

On a side note: If you ever have to defend a castle wall, Aard can send enemies flying off the edge :)

If you are low level, you can also use traps and bombs. The Conflagration traps can bring a number of nekers down to half health; Samum bombs can replicate the effects of Aard.

At any rate, whether you use traps or signs, one important thing is to make sure you at least have one level in Footwork, and one in Positioning. If possible, max them out to reduce damage taken and increase mobility.

As you get farther in the game there is a talent slot on your skill tree that allows you to damage multiple foes with one strike. The skill is called "Whirl" and is under the swordsmanship tree.

Until you get that ability I've found that the most effective way to battle multiple opponents is to dodge often. Use the spacebar to manuever around the enemies, striking occasionally as they try to keep up with your movements. Unfortunately, that's not much better than your current run-hit-run strategy.

In my experience, the 'Whirl' skill helped the combat to feel a little less sticky.

Edit - Forgot to mention this. Your Yrden sign can also be really helpful in multiple foe combat. Cast a couple of those around the area that you're fighting in and some of your enemies will get trapped and become incapacitated for a certain amount of time. This will definitely cut back on the number of enemies trying to attack you from behind.