Monster Mash (1)
Check the valley behind Ellie's garage in The Dust and kill some spiderants there. Collect their parts then hand them over to Zed.
Monster Mash (2)
This time you have to collect four Rakk parts. Shoot them down and collect their parts to update the mission. You'll be required to collect skag parts this time. You can kill any rakk or skag to get their parts for this mission.
Monster Mash (3)
You'll have to hunt down 20 skrakks. You can find them in the Arid Nexus – Boneyard area. This area will become available after completing the story mission Toil and Trouble. Head to the marked area in the Arid Nexus and kill the flying hybrid of Skags and Rakks. Use a machine gun mounted vehicle here for easier kills.
After killing the required number of targets, return to the fast-travel station and head to Frostburn Canyon. Head to the marked area and find Dr. Zed's Abomination. This is a really easy target so just shoot and destroy it. Turn in the mission to Dr. Zed afterward.