I always use follow strobing for "At the Enemy's Gates".

1. Turn off follow before mounting.
2. Send all slaves to mount first
3. Have your main mount up last.
4. I have F5 set in my keybindings as "Target Party Member 1" for all slaves and alt-F5 as a focus target. So F5 a few times so they all target then Alt-F5 sets the new focus. It should show up as the vehicle.
5. Ctrl-Insert is my button to turn follow strobing back on.

I have the below macro on F6 for everyone so they can just all spam away

/assist focus
/click [target=vehicle,exists] VehicleMenuBarActionButton1
/click [target=vehicle,exists] VehicleMenuBarActionButton2


I am using the latest Jamba-Dev release.
I use the same method with the "Vile Like Fire" quest and the one at Wyrmrest Temple. And in TotC.