What is the best location to get the most use out of the Servant Sacrifice/Brainjack combo before having a Flying Skell?
The Everlasting Millesaurs around lakes in Primordia.
You have to engage in combat with the lower leveled monsters around the Millesaur and get a purple soul voice. The Millesaur will usually resist Brainjack, but it will always hit with the soul voice. After that's successful, use Servant Sacrifice and get up to 9999 EXP. It may take a few tries if Servant Sacrifice keeps missing, so you might want to up your accuracy if possible