How do I kill them? What are the properties of "spirit" and "spectral" enemies? I'm in the Druid Grove and I just made it inside where there is, after two normal trolls, a spirit and spectral troll each. I see Jaheira as Bear-Woman doing damage to them. I just signed off after dying by them a second time. I'm sure the battle is winnable, but I googled spirit/spectral trolls and nothing obvious showed up.

They are immune to being immobilised or stunned, immune to non-magical weapons, they regenerate health and cannot be killed unless by fire or acid (they simply collapse until they regen enough health to get back up), while invisible they cannot be targeted by spells. Other than the invisibility and the fact they are much faster, have more HP and regen more quickly, they are much like other trolls. Fire and acid. Fire and acid.

What I found in one of the walkthroughs

Spirit Trolls require magical
weapons to hit (like Spectral Trolls) and begin battle with an Improved
Invisibility effect on them. They also drain Strength every time they
hit-like a Shadow. The effect is temporary, but annoying.

my advice

1. use one of the most important spells in the game

True Sight

the battle in the troll mound is a big one and the trolls will come from all directions so you need this spell to dispel their invisibility every round

2. if you can do the troll mound little by little, its difficult but it's better than facing all the trolls in there at once

3. summons to keep the trolls busy

4. I think protection from normal weapons works against trolls but I'm not sure

5. in combat finish the trolls one at a time because they have exceptional regeneration

6. weapons with fire/acid damage
flail of ages, that +3 fire axe, that acid club etc. fire/acid arrows etc

7. aganazzar's scorcher, burning hands very useful to finish multiple trolls if your targeting is good, your fighters have a lot of hit points, so don't be afraid to take a little damage in order to end the battle quicker

8. at your level melfs acid arrow last many rounds

9. buff up well before the battle

It's worth noting that trolls are actually two different creatures with different characteristics.

Form 1: The troll is active and (usually) attacking you if you're around. In this form, the troll is vulnerable to most forms of damage, but has a minimum HP item and therefore can't be killed (in this form) by damage; when the minimum HP is reached, a script will change this troll into form 2.

Form 2: The troll is unconscious. In this form, the troll loses the minimum HP item, and therefore can be killed, but becomes immune to most form of damage. After a while (I am not sure if there is any actual HP regeneration going on in this form), it will change back to form 1 and instantly recover all its HP. (Does the invisibility of these trolls get re-applied when this happens?) Most trolls, of course, can be damaged by fire in this form (but, of course, I believe Watcher's Keep throws a curveball at you at some point).

So, to kill trolls, there are two options:

1. Kill them in form 1. HP damage will not help you here; you need to find some other way. The most common way to kill minimum HP enemies like these is to use Shapechange to turn into a mindflayer, but you probably don't have that option (or you might, but it may require reading a rare scroll and spending a one-time only wish, and I don't know if you want to do that here). You could see if any instant death attacks can work on the trolls, or you could try other forms of ability damage.

2. Kill them in form 2. You need to do damage to them in form 1 (by any HP damage attack they're vulnerable to), and then hit them again with fire or acid. Having a mage cast Melf's Minute Meteors can be helpful; just make sure you explicitly target fallen trolls as soon as you notice them. Other fire attacks can work, including the Flame Blade spell if you need it. You might want to save those spells for when the trolls are actually down (in what I called form 2 earlier). In particular, due to the way the scripting works, I believe Melf's Acid Arrow won't actually kill a troll unless the troll is already down when the initial spell hits. (On the other hand, I believe this is not the case for spells like Incendiary Cloud, which affect an area rather than a creature.)

Also, I believe that polymorphing yourself into a Gnoll (via Polymorph Self) is another way to get a bit of fire damage.

Best way is to equip a wand of fire, drink a potion of speed and circle around, scorching them with agannazars. Will need a few charges.