Enemy strays too far from rift, causing restart

[versaith]

Probably an old question/complaint, but twice I've neared the end of one of the fade rift battles and in Frostback had an enemy fall off one of the cliffs...forcing me to start over because he ends up roaming around too far from the rift....I instructed my main to hold position along with one of my other terds  and it didn't work. Is there anything else I can do? Thanks!