I keep trying to get in a solid post here, & I'm having a hard time. I'll have another shot here.
So before we all separate to take position, Percy will cast a mage armor spell on Rodimus. (Say T-15 minutes)
Then Percy will line up with the Treants & Nars behind a tree, as close as possible to the fort. Percy will make sure she has a view of the Northern gap in the wall. The nomads will take up various positions where they are somewhat concealed and able to practice their archery.
Nezrak, Gorefoot, Rodimus and Ridik will lead the thorns around from the North, and take position with their backs to the cliff, out of sight of the goblinoid sentries on the wall.
Fridley will give a loud hoot. This is the signal (T). Percy will then cast invisiblity on Nars (T+1 rounds) and Nezrack will cast his five spells (T+1 to T+5 rounds): Enlarge Person (+2 Str, -2 Dex, -1 AC, 5 minutes) and Cat's Grace (+4 Dex, 5 minutes) at Rodimus, and Protection From Evil (+2 Deflection to AC, +2 Resistance to Saves, 5 minutes) at Rodimus, and Gorefoot. Percy will cast Mage armor on herself. (T+2)
Nars will begin moving on T+1 right after Percy turns him invisible. He will sneak in the gate, or through the broken tower, or wherever seems best to him.
At T+5 after Nezrak's spells are cast, Rodimus and Gorefoot will charge towards the gap, Thorns in tow.
As soon as Percy sees them approach the gap, she'll clear it of enemies with a fireball (T +6 or so). All the nomads and such launch their arrows at this point, the Treatns charge with their living trees, the petal and pixie make michief etc.
How's that sound?