"Deal damage to Laser Designated enemy vehicles using lock-on weapons"

So the point how to do that challenge is 
Recon should use laser designator with RIGHT CLICK(Binocular)
DO NOT PLACE IT on the ground
recon marks the target
engineer shooting with stinger(even on tanks etc)
ggs
P.S
u not gonna get any points if u gonna shoot with stinger on marked target if the target was marked by recon who placed SOFLAM on the ground
so grab a friend and tell him to use binocular variant of soflam