Only the minion in the current skirmish phase you are resolving is "skirmishing", the others are merely assigned. So
Ancient Roads and
Ranger's Cloak only apply to the current skirmish, and
Arrows Thick in the Air would only affect the minion currently skirmishing.
Regarding
Gondorian Prowler, he says he can make the site number of any minion +2, so he could make one of the minions not skirmishing +2 using his ability, in preparation for
Arrows Thick in the Air, but this effect would only last throughout the current skirmish, so when you transition to the other minion's skirmish, its site number would have reverted. This is because skirmish events and abilities last for one skirmish phase, and each minion's skirmish is a separate skirmish phase.
So
Arrows Thick in the Air works best against naturally roaming minions, although effects that make minions roam until the Regroup phase (
Anborn,
Second Level or
Madril, DoO) would also help this card.