On this page of the game guide to Borderlands 2 you will find a detailed walkthrough description of all of the three parts of the Monster Mash side mission. You can start these quests in the Sanctuary location after completing the main mission named Where Angels Fear to Tread.
2) Spiderant nests
Arid Nexus Boneyard
3) Skag group 1
4) Skag group 2
5) Skrakk nests
Zed  will order you to collect parts of bodies of certain animals inhabiting Pandora. Start off by travelling to The Dust by using Fast Travel. Head north behind Ellie's garage to find the passage to Spiderant nests . Keep killing them until they drop four Spiderant Parts which you need to take to Zed .
In the second part of this mission you will have to head to Arid Nexus Boneyard. If you haven't activated Fast Travel there, move to Eridium Blight and in the south-east part of the map, look for a bridge leading to Arid Nexus (you still need to complete Toil and Trouble to gain access to this passage). After getting there, right to the left of the passage  you should see a group of Skags. If you're lucky enough, you might get the required amount of Skag Parts from this group alone. If not, head to the next spot where Skags can be found .
After ending this part of the mission and accepting the next one, you will have to once again return to Arid Nexus Boneyard. Your objective will be killing twenty Skrakks which will start attacking you as you approach the area marked as .
After completing this task, return to Fast Travel and move to Frostburn Canyon. Follow the dots on the map from Fast Travel to the destination point . Here you will have to face Spycho. Look out for the shock wave that it can create after raising its limbs. The creature is very resistant to Corrosive type attacks. After defeating the enemy, you can return to Zed  to collect your prize.