I'd make a gaser (Gamma Amplification by Stimulated Emission of Radiation) and fuck up their DNA. I'd do it just enough so it took them a long time to die, and there would be no connection to me.
I would use chinese water torture for 3 days before switching the water to HCL and putting it over their eyes and then make a small cut over their stomach and have the acid burn through their skin and mucle to reach their stomach lining and then cut that so they expell more acid into there body. make more cuts in their skin so the acid seems in. and an hour after that start savagely beating them with a baseball bat in non fatal places and then leave them to do while i fucked their wife/girlfirend in front of them.
I would take a cheese grater to their feet and slowly slice it all down slowly until i hit the unshredable bone. i would then take a shoelace and shove it thru their ear until i get it to their nose and pull it thru. id then pull the shoelave until it carves its way thru the persons head, as they suffer for air. then take a potato pealer and peal all skin off their body. after all that id roll them in a pit of sault and later dispose of them in acid