The Arena is a optional mini-game that can be found under "New Game" in the main menu. To win this acheivement you must win 30 rounds of fighting against various groups of enemies. The achievement will pop as soon as you defeat the 30th round (1 Draug, 6 Draugir).
You do NOT need to win 30 times consecutively, if you lose a round, you can still continue, you just no longer earn points towards the leaderboards. However, you cannot save in this mode, so all 30 wins must come in the same sitting. (This probably took less than 2 hours)
After winning each fight you are given a choice of 3 rewards (weapons, armor, mutagens, etc), plus some orens. Win or Lose, you also earn one character level at the end of each fight.
Hints & Tips:
- Play on Easy, as difficulty does not affect the achievement.
- Before entering the arena, inspect the available rewards and compare them against your current equipment, so you know which to choose ahead of time.
- When choosing rewards, try to maintain a balanced group of equipment but focus on upgrades to the steel sword early and then the silver sword later as you will fight more human enemies in the early rounds and more monster enemies in the later stages. See a list of rounds here:
http://witcher.wikia.com/wiki/The_Witcher_2_Arena - As far as skills go, I focused on the vigor skills in the training path (2 in vigor regen, 1 parrying, 2 Fortitude, 1 Hardiness) and then got Level III Aard sign right away - being able to push away lots of enemies is extremely important. Following that, I pushed into the swordsmanship tree: Feet work, to dodge farther; Whirl, to damange adjacent opponents; and then 2 levels of invincible, finesse, combat acumen and whirlwind will make you a beast of a witcher. After that I would recommend going back to Magic and getting 2 levels of Magical Vigor and Enhanced Igni Sign.
- If you have trouble with one of the fights, you can buy some potions at the store (to the left of the arena entrance) and then hire one of the 3 companions as a decoy. (human knight is probably the best). Potions will last multiple fights, if you're fast enough.
From my experience, you have to be a little bit more careful at the beginning but you will quickly get equipment that far outclasses anything the enemies can bring (around the time you get the Kayran Carapace Armor, or similar), after that your best bet is really to just mash the attack buttons as fast as possible and use the Aard sign if you get trapped between a bunch of enemies.