I would think on a 1-to-1 wyvern riders wouldn't be up to it alone...
Thus...
- Nasty wind-surpressing magic (maybe just a big "no fly zone" for Orlanthi
magic)
- Young-elemental sylphs with an eerie red glow
- Intellegent and very large birds of prey
- Nasty Star Magic that zaps flyers at dawn and dusk (to flyers what "Star
wars" was supposed to be to ICBM's)
- Dara Happan angel guardians associated with ritualistic mumbo jumbo
(yowch)
General drift would be to get away from the idea of a one-to-one
confrontation and instead come up with a general solution that compromises
Orlanthi flying on the whole and period. Wasn't there something a bit like
this in the Cradle scenario that just cancelled everybody's magic as the
Cradle crossed some kind of magical line?