Just finished one hell of a dark mode session in Arena right now, so I'll give you some strategies and tips. Dodging is an absolute must. Sometimes, I like to bounce off different enemies, striking them once and backing away to a different one. If you have bombs, please use them and try to upgrade bomb+trap damage in the Alchemy path. The upgrades that increase damage are phenomenal because you have access to all the herbs and can craft bombs in bulk. These are very useful for thinning out crowds and mowing through large groups. Traps are also useful, but I find their tactical value lacking because Geralt takes time to deploy them and often you won't have the chance to parry in enough time to save your health. Throw bomb, dodge if necessary, lay a couple of strikes and be ready to throw another few. My favorite bombs are Grapeshot and Dancing Star. These two will make you a killing machine if you gather enough herbs and buy a lot from markets.
Secondly, use your potions a lot, especially when you are at a low level venturing in the forests of Flotsam. I use a combination of Swallow, Golden Oriole, and Tawny Owl (for its vigor regeneration abilities) or Wolf for some critical effects that could potentially help. Try to invest a couple of points into the talent that increases the length of potion duration. It is very useful since you'll be somewhat protected from any attacks by poisonous monsters. Moreover, if you couple this with Quen, you can regenerate your health in the most dangerous of times (especially when you managed to get hit by Endrega Warrior or Nekker groups). Other potions that have some potential include: Rook (for the 10% damage dealt to all opponents) and Virga (a suitable replacement for Golden Oriole since you have the 20% armor bonus and increased defenses against incineration and bleeding)
Talking about Swordsmanship, gather enough ingredients and money to buy the Jagged Blade once you're in Act 1, as it deals the most damage to all human opponents. I like to parry a couple of times since Geralt's dodging abilities are at the lowest abilities when you first start out and mix in some Aard or Igni to deal small amounts of damage. This will give you some time to attack from the back and pull off some strong and fast attacks. But once you have points invested in dodging, parrying is almost never used since you can easily kite around enemies and lay killing blows most of the time. However, I will say that Scoiatel Swordsman and the Kingslayers in Arena are notoriously difficult because they parry SO much. In this case, I would parry and do a riposte, if you have the necessary points for it. Still, it's easier to dodge and use your bombs+signs to deal significant damage. You can also use Yrden to stun an enemy for a while, enough to land blows and prepare for the next maneuver.
One last thing! I also stress using the right oils against monsters that you have knowledge of. For example, I was having trouble with Melitele's Heart fight against the wraiths, since they take an obnoxious amount of hits to take them. I used Specter Oil to deal much more damage, in addition to the set of potions I usually take anyway, and could comfortably land strong blows to deal significant damage. Many times you're sword will deal so much damage that using oils is not necessary, though Whetstone does help to give your strikes a little more power. The earlier levels are frustrating if you don't use all these tools at your disposal, as I was at times.
Somebody mentioned using Quen+Axii and that is very useful, if you manage to use Axii before your Quen dies out. If you can control a big, armored guy, you can certainly make fights easier and more rewarding in the end.
Have fun!
