Each group of ten remains invisible, flying, and firing barrages of arrows at:
(1) Groups or individual wizards
(2) Obvious clerics and healers
(3) Front-line combat warriors, knights, fighters.
Depending on what else that you have going, such an ambush can be quite harrowing for the party. Toss in two groups of six or ten 18th level wizards that all agree to drop targeted fire-power on the group of wizards or clerics, and then have a phalanx of mounted knights or heavy infantry rushing in with halberds...