You can get this mission after completing "My Life for a Sand Skiff" by checking the Oasis bounty board. It turns out that Ms. Aubrey Callahan III has some Stalker problems and would like you to kill one for her.
|Normal||True Vault Hunter|
After getting the mission, you'll actually get a waypoint that will lead you directly to the stalker you're looking for. There will be a stalker nest with a bunch of Tri-tail stalkers and a Badass Stalker. The Stalker that you are looking for in particular is a rather large Stalker called Tinkles. All of the stalkers are capable of ranged attacks as well as the spinning motion that reflects your shots so be careful.
If you seem to be having trouble with the stalkers, run into the nest to get their attention and then retreat back to the Sand Skiff. You may be able to lure them far enough back that you can use the Sand Skiff to kill them rather than conventional weapons.
Whichever method you choose, once Tinkles is dead, you can head back to the Oasis bounty board for your reward.