Preparation is key, and strategy is another. Don't rush into the Black Forest; take your time in the Meadows by getting proper (leather) armour, weapons, and food.

Trolls can be very easy if you use a bow, as you will be able to stay at a distance.
Skeletons are weak to blunt, so a club (or even a torch) is the best against them. Stagbreaker works, too, but is slow.
Never let the greydwarfs swarm you. Running around is necessary early on, so that they cannot surround you. If you have a torch in either hand, regular dwarves will only use their rock throwing attack.
You can also use the environment (e.g. rocks and structures) to your advantage. Enemies cannot climb well, but you can. Trolls can destroy the stone towers, but they cannot destroy dungeon entrances.

Proper food is essential to keep your stamina up and health high enough to survive a hit.

I've played single-player way more than multiplayer, and find it more enjoyable in some aspects. Certainly more challenging, but not unreasonably hard.