For those of you who may freak at finding yourself with no trees to teleport by nor ikrans/banshees to travel back to past levels (for those more elusive clean sweep achievements), you'll be relieved to know that world travel will resume after you clear this stage. Some may still freak out at the loss of some other conveniences though.
The last stage is a trek for the player who has been relying on recovers and/or bow and arrows to help survive the RDA swarm. ;> There are none. Plants don't give cell samples nor arrows this stage (at least not where it matters). Fortunately, respawning at checkpoints don't seem to directly respawn enemies (they respawn on their own timer), so you can save recovers for when it matters to you. But you will end up having to use your arrows very sparingly.
Best bet is to choose the armor with the most protection this round and keep whacking using your strongest staff, making good use of your evasive flips as you go. (Clubs are good for damage but are way too slow to survive an onslaught.)
At the village midway, where a team of RDA soldiers invade, don't be afraid of the timer once you've dispatched the 18 soldiers it required of you. The timer is just indicating how much longer you have to hold out to clear this defense objective.
Once you clear the village and fly (in large style ;) ) over to the Well of Souls, you will be treated to an impressive but short activation scene and the popping of this achievement. Congratulations!