hong said:
20th level characters can usually afford to take their time.
Re forcing PCs to 'run away' - it's not the physical space that counts. What happened in my example was that the PCs took shocking amounts of damage and withdrew from attacking this small scout force. I'd expected them to win BTW - it wouldn't have mattered much, there were another 200,000 horse archers on the way along with their own NPC Archmage (Hakomon), high-level Fighters, Clerics etc. I was surprised how easily the PCs were defeated by a scout force.
I certainly wouldn't claim it was impossible to win this battle, merely that it was possible for not-totally-incompetent 20th level PCs to lose it.
The PC Archmage was indeed particularly unsuited to this battle, since Abjuration was an opposition school to him he had no prot fr normal missiles, so it barely mattered that the archers had some magicked arrows.