I did a lot of thinking on the battle these past few weeks... turning it this way and that. I made some charts. Erased them. Made more, erased them too - nothing quite captured my thoughts, so I'll try to explain.
I think I counted 5 occurrences of the classic "here's the cavalry in the nick of time" in a two-chapter span (Circa where the goblins show up). Knowing what I read about how she gathered up allies in the past 10 or so chapters I realized the pattern after the 2nd occurrence. It creates a vibe of "Rhiannon can't lose because there's always going to be somebody there to rescue her."
Ack! New danger! - Phew, someone showed up to solve it
Oh no-- another problem! Oh wait, that's solved too.
Might I suggest you have Rhiannon more actively aware of her allies motions through the birds and reserve the "surprise cavalry" for only the last entrance? If Ellsbeth could also talk to the birds you could give her the role of brilliant tactician.
An alternative (and this sounds like too much work) is to have her link up with her allies and form a larger allied force. Except the one you'd really (really) want to be the group that saves the day in the nick of time.
Also you are missing a Rosencrantz and a Guildenstern who will be important to the MC but will find a way to die yet the MC is too caught up in the events to mourn / care / plot-around. I recommend Ellsbeth for one. And you might introduce a shield bearer that MC likes, but gets taken out by a random arrow.
That was Garth in Elf Queen of Shannara or the father-son pair in 300 (I forget their names). These characters imprint on the reader but are not MCs (Ok technically Garth was) so they can die at the 50% mark and on and it's understood by the reader that's why they're there. You could also get rid of an MC. It seems painful, but hey... Tasha Yar... Obiwan... the list goes on.
Lacking these two, Rhiannon enters the largest battle in her history and comes out with 100% of her close circle intact. Yes, we lost a lot of nameless characters, and we lost civilians, and yes those losses affect the MC. But they don't affect the reader, and they don't build pathos.
So you can see in in other writing, let me point out "Alan" who dies in the spaceship Laurie blew up. I would venture it is not possible to generate pathos for him (or for her) because he's only a name. To elicit even a note of sadness, he must do much more to imprint on the reader (talk about his dreams? Wife/kids? Rescue the MC?) then still manage to die.
Note: I have taken it for granted you want the reader to feel loss as a result of this battle.
Returning to the battle, amalgamating forces would logically shorten the fight, which is probably the opposite from what you intended, but it could (with a lot of work) be accounted for. If you put Seidel's main town on a hill and give him surface-to-air attacks, he should be able to hold his ground a little better and make MC have to spend a few pages in siege. Storming and breaking walls would more than gobble up the page difference, plus it's a change of pace from the flat-land melées that dominate the current scenes.
The MC enjoys too much air superiority (flora/fauna/sparrows/etc, harpies, dragons). This isn’t an equally matched war... it’s US vs Iraq where the good guys can just sit in the air all day. You should give Seidel the dirigible flotilla and the flaming ballistas. Make it so the good guys can’t envelop/suffocate him.
(Also you have two dragon characters but only seem to need one. Also, you need more named deaths. *wink*)
I do like the relative simplicity of the battles (in terms of relatively flat terrain and no zaniness such as turn-coats or undead-ninja-assassins). No Legolas - that’s a bonus. Just two armies coming together in a bloodbath. Don’t lose that if you take any of these suggestions.
As usual, YMMV