I use /target instead of /assist and have targettarget in my macros, I don't experience this problem with fear bombs, however I am also constantly reaquiring the target in combat.

You can, use /targetlastenemy [dead,combat][noharm,combat] in spell macros to automatically retarget after losing a target. It will check and see if your target has died, and target the previously selected enemy, this is good for kill order too. Or if you have nothing selected that is a valid target for the spell, it will target the last enemy you had targeted.

The kill order adavntage---target the second guy you want to kill, then target the first guy you want to kill. Have followers assist, and burn target down, once target dies, you will target the first guy you picked earlier, and slaves will help you kill that target.
Disadvantage---fears will switch the first and second target, they will still be the first 2 to die, but if this is too much of a problem for you remove the [dead,combat] section and it shouldn't have that effect any longer.