I was browsing through my WotR book, when i realized something. If you want to field say, a troll army, you could do it because you only need as many common formantions as you do rare. you could have one 15 pt common company (as a formation) for each 100 pt mordor troll you have, and then you would have your troll army. i dunno what you would take as a leader for it though, its only an example.

any thoughts?

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One ring to rule them all, one ring to find them, one ring to bring them all and in the darkness bind them

They are easy kill points!

And take Bhurdur as your leader

If you want it to be effective make sure the one-company common formations have high defence (or resilience).

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

The key problem is that you need a common formation for each rare formation as well as a common company for each rare company, which General Elessar has touched upon. Other than that, if you have cheap and weak formations with nothing but trolls, you are likely to take a huge beating, but it may still be fun. However, it may also be very boring, lol.