Location: Fangorn.
Strategy: The basic idea is that killing trees in Fangorn works much the same way as trying to kill citizens inside a city, with a few notable differences.
Wanted status: Trees and Ents count as citizens. Like in any other city: attacking or killing a citizen is a crime and if there is a witness to that crime that isn't fighting, it may report you. Aside of that camping and using fire or lightning magic is considered a crime too. Camp fires seem to attract trees who then make you wanted if they find you hanging at the camp fire. Once you are wanted you will get several warnings to leave the forest and then become wanted dead at which point any tree you run past will start to hunt you. They can cross water.
However, until you are wanted, you can kill the trees in Fangorn without much difficulty. While they will assist each other, they are usually solo, and do not hunt.
After you have attacked or killed a tree, you need to start being careful in your movements through the forest. The longer you stay in a room with any tree-mob, the more likely they will be to inform you that you are (or will become) wanted. It does not seem to matter if there were any witnesses to this crime or not. However, there is one way (besides avoiding the tree-mobs) to avoid getting flagged wanted: while a tree is fighting, it will not advance your wanted status. This means that if you only attack trees that are solo, and always hit them instantly upon entering their room, you can delay getting wanted for a long time. (Note: this is kind of a mix of untrue and inaccurate information. Clarification: Thing is if you aren't reported yet killing only solo trees helps you remain unreported. Then it doesn't matter how fast you hit them. If your crime was reported, then hitting trees instantly prevents them to give you warning about leaving forest - as they can only do so while not engaged - hence it allows you to stay in the forest bit longer (you become wanted dead with the 5th warning) - this works in other cities too, only the warning is different. Of course if you are already reported it doesn't matter anymore whether they are solo or not, as there is no difference to being wanted for 1 or more crimes in Fangorn. /Elestir)
Also, note that one aspect of city justice works much the same in Fangorn: if you are fighting a citizen (in this case, a tree), flee from it, and re-enter its room, it is very likely it will advance your wanted status. (Clarification: that's because the victim itself is a witness, and the only reason it didn't report you before was, that it was prevented from doing so because of being engaged in combat. /Elestir)
Once you have become wanted dead, it becomes very difficult. Not only will the trees hunt you, but if you start fighting any tree, there is a good chance that trees nearby will converge on you.
Alignment of trees: If you are a BN, you need not worry about alignment loss.
If you are playing a good-side character, stick to trees with keyword “huorn” if you want to avoid losing alignment. The giant yew, haughty elm, mature linden, ancient oak, and black alder trees all have this keyword, and are safe to kill without any drop in alignment. Aside of these, also tall ash trees are killable and non-good.
In addition to these, there are several more trees that are “borderline.” These trees are very slightly good aligned, so you can kill quite a few before you see your alignment drop visibly. The slender hornbeam and tall scots pine fall under this category.
The imposing beech tree and birch trees are significantly good aligned, so do not kill them unless you want to spend a lot of time fixing your alignment. (The same goes for the named Ents, obviously.)
Other information: There are 12 different types of trees that are solo-able in Fangorn if you are a BN, and 6 (or 8, if you're willing to lose a bit of alignment) different types if you are playing a good-side race.
The trees are stabbable, but realize that stab/fleeing is likely to make you become wanted rather quickly.
A solo mage with a charmie (mother eagle works quite well) can solo Fangorn easily. Have the eagle buff, and trees die in 4 fireballs plus change. Since trees start off being worth upwards of 20k apiece, this means that for a BN, Fangorn is an easy 3M exp or so (approx. 200 ents).
Do not try to solo Beechbone, Treebeard, or Leaflock. (Treebeard required 68 fireballs and a bit under 2 hours for me, yielding approx. 160k xp - Scad)
Some trees switch, and most are not bashable. All trees have a mudlled bash attack which can damage your sword and shield, and can make you unwield/remove them.
There is exactly one tree in Fangorn that you can kill without any risk of getting wanted. A bit northwest of the Ford in the River Limelight (at the northern border of Fangorn), there is a hidden exit north named 'foliage.' Behind this is one black alder tree mob, one black alder tree item, and a clump of roots. This tree seems to be outside the 'justice' system of Fangorn.
MANY THANKS to Kvetak, who provided most of this information.
Level: Hero mage or cleric with charm, or legend thief.
Mobs unique to the zone: