I've played through the demo as Sword/Block, Dagger/Dodge, and Axe/Block. I also became Arena Champion with a Crossbow/Critical character, but couldn't fight my way through the squad of imperial Guards that payed me a visit at the inn. Currently playing a Sword/Critical guy and having big trouble with the two Ordu. Damn archer keeps shooting my legs.

Based on my experience anyone wanting to beat the demo needs to focus on two skill. The first is their chosen weapon and the second is one out of Dodge, Block, and Critical. Out of those three I find Critical is the hardest to play through with, especially in fights against multiple opponents with good armor. There are two specific builds I find especially easy.
1) 10 Dex character with Dodge and your weapon of choice. Daggers work well for this since your Dagger skill starts as 4x Dex, but other weapons also work. The main benefit is that you have lots of action points and you can choose between using a 2-hand weapon or a 1-hand weapon and a net. And you don't need to worry about losing your shield to a an enemy using an axe or large weapon.
2) 7 Dex character with Block and a melee weapon. Hammer and Axe go well with a high Strength. The main advantage is that you can wear the heaviest armors and you should buy that bronze armor for 200 gold (I think it's Praetorian armor) as soon as you have the money (I can afford it after the third fight). It's the only armor you will ever need to buy as it will serve you well until you can take something better of the fallen corpses of the enemies you've beaten. And ultimately you'll be wearing the upgraded Imperial Armor that you get from the Imperial Guards' commander.
Either way, you'll want to pump up your defensive skill as high as possible. I don't put any points in my melee weapon skill until my defensive skill is 150 and after that I'm still putting two thirds of my points into defense. By the end of the game you'll want a nice 300 in your defensive skill, which will leave your weapon skills somewhere around 160.
Weapon+Critical builds on the other hand will want to focus on their weapon and only invest in Critical Strike once they have a good chance to hit even when aiming for specific body parts.
Starting values of weapon skills are:
- Dagger = 4x Dex
- Spear = 3x Dex + 1x Str
- Sword = 2x Dex + 2x Str
- Axe = 3x Str + 1x Dex
- Hammer = 4x STr
- Bow = 2x Dex + 2x Per
- Crossbow = 3x Per + 1x Dex
- Throw = 3x Dex + 1x Per
Note: the demo also gives you a 50 points bonus to the skill you select at character creation on top of these values.
And for the other combat skills:
- Block = 2x Dex + 1x Str
- Dodge = 4x Dex
- Critical Strike = 2x Dex + 2x Per
And one handy thing to keep in mind is that when you're fighting multiple opponents and you kill one of them you can press "c" to enter the character menu and spend the skill points you just got.
In regard to Power Attack vs Aim:Torso it depends on the weapon you're using and the armor of the enemy. The weaker your weapon and the higher the enemy's DR, the more it's sensible to aim for the torso. Against the Imperial commander with his monstrous 12 DR it's usually pretty useful to aim for the torso if you have a 1-handed weapon since you'll be doing 6 more damage than with a normal attack. Not sure how much power attack improves damage but I'm quite confident the improvement is smaller and you also suffer a penalty to accuracy.
One trick that is pretty neat against enemies with 2-handed weapons is aiming for the hands to disarm them. They'll have to spend 4 AP reequiping their weapon so they'll only be able to make 1 attack against you instead of two and that means half less damage. Works best with a quick weapon though (4 AP or less for normal attack) or else you might be robbing yourself of the chance to make a proper attack that turn.
I probably don't need to mention aiming for the legs if you have a spear or ranged weapon.

Headshots in general are a bit of a disappointment. I haven't noticed any special benefit, except when an enemy has strong body armor and no helmet.
Generally the demo is pretty fun, even though frustrating at times (like losing a battle 8 times in a row). I'm slowly begining to loose interest in the arena fights themselves (beaten them with 4 different characters), but I'm not quite finished with the demo yet.