If you have the Slaughter Dome DLC, then it's a little easier. You can just do the one run until you get to the area they spawn at them run Into the slaughter dome just inside the building there and save and quit. Reload and walk back into the wildlife exploitation preserve and see if they spawned. If they are the hope they drop their legendaries and if they aren't walk back into the Slaughter dome and save and quit again. Just keep doing this and you won't have to run through the whole area to get to Tumbaa and Pimon.
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