If you've tried this mission, you will most possibly find that at one point (after the scorpions) the chicken will run into the mountain and disappear.
Well the way to avoid mission failure is that when the chicken starts running, quickly get into combat with the mobs. The chicken will come and help you, just drag the mob to somewhere north before kill it. After a few kills the chicken should act normal (start walking instead of running) again.
And there's a second bug point (after the humanoids and close to the end) where the chicken run north quickly. Do the same thing to complete the quest.
Took me five runs to complete it. And better make a lvl 50 team cause the other factions' 60s may assault you.