Ah, I am still having serious trouble getting my ?-8 completed.
I've followed all the advice here, starting all in top or bottom camp. I even rearranged a special squad to get all their speeds as close as possible. No luck still.
The last thing I am going to try is to see if I get a helper with really low speed, if that will make a difference. The past two runs my helper actually takes the portal to the lower right and beats the boss, but before my squads can either attack or finish off the top boss, she makes it the flag.

Close, so very very close now.