If the range is at 5k and there are no enemy rainbows:
Send scouts/ponies to layer as well as ranged units. The ranged units might catch up in time to give your HBs an extra round or two of safety, but don't rely on it.

If there range is at 5k and there is an enemy rainbow:
Jus' send scouts/ponies; the archers won't ever reach the AT range in time to be any use

If the range is AT range:
Send archers, b'listers, catas, scouts, ponies to layer up; they'll all take the hits before your 'phracts do and give you as much time as you need