Playing as Salvador at level 68 on PC I was going to try and kill Pyro Pete. No matter how many times I tried I could get him to appear. Do you have to finish the whole DLC before you can take him on? Currently I still need to complete the Motor Mama, The Forge and the fight against Piston. Is this why Pyro Pete isn’t appearing?
You have to pay eridium in the area he spawns in to make him appear.
Yes, you need to finish the story of the DLC first. Moxxie will then have a quest for you to fight him and after that quest is done you can spawn him in with Eridium to farm him.