Hello. I made a calculation mistake previously, so I edited this message. So, infrantry is more expensive than I had previously expected. But combining Rohan with Fangorn asks for using infrantry because Rohan looses their S4 cavalry anyways.

So, here is a new attempt. Not best on earth, but I tried:

Fangorn + Rohan (697p), 10 might, 34 units, 15 bows, 1 monster, lots of throwing spears.

*TreeBeard (leader)

*Theoden King of Rohan with a shield
-6 royal guards of Rohan with shields and throwing spears
-10 warriors of Rohan with shields and throwing spears
-2 warrior of Rohan with bows

*King’s Huntsman
-6 warriors of rohan with bows

*Dernhelm on horse (Gondor at War rulebook)
-6 riders of Rohan

Would this army have any chance in a competitive scene? How should I improve it? Would using Full Cavalry be wiser than this infrantry based army even without the army bonus? For full cavalry i could get Theoden, Dernhelm, Treebeard, 17 riders of rohan and 6 royal guards on horse with throwing spears. Thanks a lot for answers.

Ok Here is a possible Cavalry version. I could add 2 throwing spears but I prefer make lists without a need to memorize specific unit items so:

Fangorn + Rohan (700p), 8 might, 27 units, 24 archers, 1 monster, 26 cavalry.

*Treebeard (leader)

*Theoden King of Rohan with a shield and an armoured horse
-18 riders of Rohan

*Dernhelm (Gondor at war army book)
-6 riders of Rohan

I'm inclined to favor the mounted lists actually. Yes, you've lost your str. bonus but you've got fight 4 when you're charging, a lot more bows, and are more mobile. The trouble is Treebeard is so slow. In a death match type game the opponent can force a fight wherever Treebeard gets caught out. Nice the rest of the army is mounted, though, you should be able to reinforce that point well enough.

As for competitiveness, honestly I'm not sure. There isn't a whole lot of might to manage all those horsemen. I don't have a great deal of faith in Treebeard being able to carry the team in hero killing and aside from trying to gun down wizards/wraiths, you don't have much of a way to deal with them. Those difficulties might be hard to overcome.

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

Thank you a lot for answer! You are right that the higher fight value and the higher amount of bows are good so cavalry is the obvious choice! Also you are right about the mages, but i guess my only hope is enemy doesn't have both archers and mages, otherwise this army is doomed.

Btw, could this army be stronger in 500p? Theoden maxed out, 15 riders of Rohan and Treebeard sound like they can play good in 500p? 500p is in fact the standard here. Sure if enemy would play like Angmar i guess treebeard would end up paralyzed fast and then gg. But I haven' rly found any interesting good factions so far i want to have some kind of "center piece" such as treebeard, dragon, sauron, etc. So should i just abandon this army list, is this rly just bad? I don't mind loosing 50% games but I don't want to loose 100% games.

I don't think the army is really bad in so far as it would loose all the time. I think you can get match ups where it would work out a 700pts. At 500pts, I think the same issues and strengths would apply. If you really like Treebeard I think it's worth giving it a go and seeing what happens.

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

Ok, i have tested the army on roll20 online platform. I must admit that Rohan plus Fangorn do not work well together. The loss of army bonus is very big hit every time rohan fights typical elite enemies with D6 or D8. Also, treebeard is very slow, and horses cannot rly support him in small space. Also the lack of banner was bad because treebeard has after all only 3 strikes and horses lost practically everyhing versus infrantry. So, despite the fun theme i guess this army needs a replacement of some kind. But if i come up idea with a new army i will start a new topic. So thanks everyone for help!
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EDITTT: Ok, despite the promise i made a new Fangorn+Rohan army list. Because Rohan cavalry looses their army bonus, and does not work very well with the ent, maybe Rohan infrantry after all is the more formidable choice? So here is the new army list:

Fangorn & Rohan (700p, 39 units, 11 might, 1 monster)
*TreeBeard with 2 hobbits
*Eomer with a shield
-14 helmingas with shields
-1 helmingas with a banner
*Grimbold of Grimslade
-12 helmingas with shields
*King’s Huntsman
-6 warriors of Rohan with bows

So, there is a banner, S4 infrantry a strong fighter-hero and an ent! Also, the King's Huntsman could possibly be useful at sniping something, such as snipe a ringwraith or an orc shaman behind an orc line?