What troops to buy to play 500-800 points?
Thranduil on Elk + Legolas + Tauriel
And troops is basically "Mirkwood Elf"? I understand "the Palace guards" is not needed?
What is the most powerful army you can gather for 800 points?

It is quite hard to say as rules went just live.
But I would try to add 5 riders as they are quite good around their leader. Palace guard are still quite costly, likewise for the rangers.

In terms of what you should buy I'd say all three heroes are going to be a must. Getting the mounted versions of both Legolas and Thranduil is also going to be worth it. I would then buy 1 box of rangers and 6 to 9 warriors with bow and 9 to 12 warriors with shield and 9 to 12 warriors with glaives. 4 to 6 Cavalry are also worth getting. That will give you a good sized set of basic warrior troops to serve as the core of your armies. Plus, the units to make use of the army bonus. From there it will be a question of building lists and finding out how you like to play the army and utilize its tools. I would not rule about the palace guard completely but they are definitely not worth more than a single box in total. They are not main line infantry and their bonus relies on keeping Thranduil in the main line. They are only going to be useful if you play a certain sort of defensively minded army with your heroes on foot. The killing power to be had from a mounted Thranduil and the support he gives to the knights is a much better choice in most players opinions.

Going of the Rivendell lists I've played I think that at 800pts your most powerful army will be Thranduil and Legolas mounted with a squadron of cavalry, a large like of elven infantry with a captain mixed it. The Taureil and a full unit of rangers to shoot stuff. I do not have the new book at present so I won't give you an exact list without the right figures for points cost but it should be about what I've described. Unless, you try a ranger spam.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

So i haven't played the new rules yet but at a glance, having played the old rules with both Thranduils halls and wood elves, I would take something like this.

Warband 1 18/18
Thranduil, King of the Woodland Realm with Elk; Heavy armour; Additional Elven-made sword; Circlet of Kings
1 Palace Guard with Spear
5 Mirkwood Elf with Shield
2 Mirkwood Elf with Elf Bow
3 Mirkwood Elf with Elven-made Glaive
2 Mirkwood Cavalry with Shield
4 Mirkwood Ranger
1 Wood Elf Sentinel

Warband 2 14/15
Legolas Greenleaf, Prince of Mirkwood
4 Mirkwood Elf with Shield
8 Mirkwood Elf with Elf Bow
1 Mirkwood Elf with Elven-made Glaive
1 Wood Elf Sentinel

Could tweak the elites around a bit, maybe trade some of the rangers for more palace guard or try to fit in another cav or two.
The other biggest option in my opinion is whether to try to take three heroes. I have no idea which is better. At this point I am leaning towards just taking a big fatty hero but maybe still hording when possible is good.

If you want to take three I might try something like this.

Warband 1 12/18
Thranduil, King of the Woodland Realm with Elk; Heavy armour; Circlet of Kings;
5 Mirkwood Elf with Shield;
2 Mirkwood Elf with Elf Bow;
2 Mirkwood Elf with Elven-made Glaive;
1 Mirkwood Cavalry with Shield;
2 Mirkwood Ranger
----
Warband 2 11/12
Tauriel
4 Mirkwood Elf with Shield;
5 Mirkwood Elf with Elf Bow;
2 Mirkwood Elf with Elven-made Glaive;
----
Warband 3 9/12
Palace Guard Captain with Shield;
4 Mirkwood Elf with Shield;
3 Mirkwood Elf with Elf Bow;
2 Mirkwood Elf with Elven-made Glaive;


You could take Legolas instead of the captain at the cost of dropping a model, might be better but then youre on the same number as the list with 2 heroes. You also could take him over Tauriel. In this list especially it might be better to take the second sword over the crown on Thranduil since if you also take Tauriel and the captain, those are heroes who want to get stuck in. I really rate the Mirkwood ranger and guard captains, perhaps one of the best captains. The guard captain is great at digging in and sitting on an objective and the ranger is a good Tauriel at a discount that you can toss in and try to rack up a nice set of dice for some wounds.

Good luck!

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Cheers,

P