Mount&Blade/Arena

The arena is intended to provide a balanced sampling of many weapon types in the game. Because of this it's not always best to quit a fight just because you didn't end up with your favoured weapon. Also if you placed a bet you'll lose your gold, whereas if you keep fighting you might actually win after all. You can't die in the arena and there's no penalty for failure other than losing your bet, so you can fight without as many worries.

Fairly often you'll start on horseback. Even if you don't, try to get one; if an enemy is knocked off his horse climbing on it yourself is a very good idea.

One-handed sword
The most balanced weapon. It trades damage and range for overall speed. You also have a shield, making archers less of a threat.

On foot
On foot you're most vulnerable to mounted fighters, so learn to dodge and use the shield effectively. Archers are especially easy to slice up; just advance while blocking and come within melee range. Eventually they'll switch to their fists, allowing you to slice them to pieces.

Unhorsed knights are an especial annoyance; you'll need to deliberately sacrifice some health in order to get within their range and lay into them. Once you've smashed their shield they're helpless.

On horseback
On horseback it isn't as effective a weapon, however the horse's speed bonus as well as the ease with which you can flee combat more than make up for this. Watch out for knights couching their lances against you; not even a braced shield can stop you or your horse taking devastating damage.

Two-handed sword
Not the greatest when it comes to defense but excellent on horseback.

On foot
Steer clear of archers, unless they're distracted. You're better off hunting for melee opponents.

On horseback
On horseback this is the best weapon. Its long range helps, especially against knights; however you're always open to the threat of archers.

On horseback
If you've still got your horse the lance is the king of arena weapons. Your turning speed may not be the best, but you can keep taking passes until you get it right. Don't bother with the normal left-click attack, instead just ride the horse until the lance drops and becomes couched; if the tip hits someone in this state you'll deal incredible amounts of damage even if you have no lance experience whatsoever.

If you miss an enemy don't slow down and poke at him. For mounted enemies you may need to use the stabbing attack, so slow down and aim accordingly. However getting them within range of a couched tip is still the ideal. Especially watch for riders that have collided with walls; they'll take a few seconds to turn around and get up to speed (especially if they ended up in the gateway) giving you a good chance to run them through.

Be especially careful of losing your horse. If you lose it, you're probaly done for.

On foot
On foot the lance simply sucks. It is slow to move and if an enemy gets inside your guard you have real problems. Archers shouldn't be that much of a concern; by the time you're within range of them they'll have changed to their fists, leaving you to either stab or punch them to death while blocking with what remains of your shield. Melee foes are a bigger concern, and once they get inside your guard you may well be done for.

Bow
If you specialise in melee combat this is the worst weapon to be stuck with. You have no shield and probably horrendous aim. The biggest threat is from those on the ground (especially if you're also on the ground).

Mounted enemies aren't that much of a threat as you can often get out of their range.

On horseback
Having a horse makes being an archer less of a problem as it allows you to flee danger easily. Just stay back, aiming and zooming as necessary. Your arrows aren't unlimited so remember that right-clicking cancels a nocked arrow. You can negate the usual mounted penalties by stopping the horse completely before shooting. Watch out for both mounted and ground enemies sneaking up and damaging you or your horse. Other archers aren't much of a problem as you can probably dodge them (and their aim isn't very good anyway).

Against mounted enemies the first thing to do is kill their horse (unless you're confident enough to get frequent headshots). This makes them a lot more vulnerable both to you and your teammates.

On foot
If there's an ally on the ground stick with him. He'll both draw some attacks away from you and also help fend off attackers.

If it's a knight with a lance you've got a chance. His weapon is slow to move. However once you're within his range he's got you; once this happens it might be best to switch to your fists and beat him up. As your fists have a quicker reaction time it's probably easier to do this than shooting arrows inbetween attacks.

If it's a swordsman things get more troublesome. Really you need to shoot them before they get within melee range, and break their shield if possible. Once they close on you nothing but speed and maybe jumping can save you. Your biggest hope is of course that a mounted ally will come and disrupt the fight.