I would spend many years slowly befriending my enemy, learning all there is too them, what makes them tick, what makes them hurt. Becoming their best friend over the years, meeting and becoming close with all their whole family. Now the life ruining begins, I would spend much time and money slowly working my toxin's into their lives, they won't know where it's coming from, nor exactly what is unravelling right under their noses, but when they figure it out, it'll be too late. I'd begin by tossing some deceit into the mix, turn family members on another, break down structure from the inside out. I'd introduce carcinogenic compounds to their homes discretely, over time remaining close members of the family would become horribly ill, adding more to the emotional stew. I'd attend every funeral, every family event, become such and involved part of their life that they can't imagine a life without you in it. I'd wait till my enemy was at his/her lowest of lows and that's when I'd fake a suicide. Everything left in their world is grey already and now they've lost their best friend, and the icing on top of this shitcake would be the lovely note left behind, written in such a way that they believe they were the cause of your death. They'd read the note, staining it with tears, experiencing every negative emotion in one raging storm and the only result is pain, not just any pain, an overwhelming, deep, burning pain that would allow no pleasure to ever make it's way into their life again.