I’m having trouble killing Letho near the end of the first chapter. How to do it

I played the first one through and loved it. I'm now playing Witcher 2 and loving it as well.

If your concern is the plot and back-story, I think you'll do fine just reading the plot summary on Wiki

As far as importing a save game, it carries over a little bit of money and gear, but nothing that gives a huge advantage. You'll have earned more money and better gear in a few hours anyway.

The biggest issue I can think of is in understanding the mindset of the character, the game, and the world. The Witchers are powerful warriors like you'd find in other games, but that's not what makes them interesting/unique. Their specialties are knowledge of the many exotic monsters and their alchemy with potions (and traps and bombs in this case).

Reading/researching about creatures would be optional codex/lore in another game, but here it is part of the quests themselves. Potions, bombs, traps would be optional consumables in another game - and I personally rarely use them - but here they're critical.

Many people are struck by the difficulty of the game, and it's usually because they're trying to sword-fight their way through everything. If you really make use of the traps, bombs, potions, oils, etc. you should do just fine.

Lastly, understand that the character Geralt is not trying to be a heroic soldier. Unlike in comparable RPGs, you're neither a great hero nor villain, just something like a tough exterminator travelling from village to village cleaning up their monster infestations. Getting caught up in politics and wars is anathema to the cynical Witcher who just wants to be on his way to the next job.

The sign that protects you is Quen, it absorbs part of the damage and prevents you from being incinerated. It is extremely useful, just know that your vigor doesn't regenerate while it's active.

In general, for group battles you should make liberal use of everything you have available. If you have time to prepare, drink a potion (e.g. swallow) and set traps. Use Axii to make one of the enemies fight on your side, distracting the other. Yrden sets up a trap that hurts and immobilizes an enemy for a short while, also very useful. And before you engage in sword combat put on Quen to protect yourself.

But the most important thing is to never get yourself surrounded! Try to seperate the enemies and use hit&run tactics to attack one enemy and quickly evade (double tap direction key or space) once the other enemies arrrive.