Estocada literally means a "stab" or "thrust". It's most famously used to describe the final, fatal sword blow delivered by a matador in a bullfight.
In this song, Ricardo Arjona uses the phrase "dejando al tiempo la estocada a muerte" (leaving the death blow to time). He uses this incredibly powerful and dramatic metaphor to say that the relationship wasn't ended by a single event, but was left to slowly and inevitably die over time. It's a unique and poetic word that captures the tragic end of the love affair.
This song is a classic he-said, she-said argument about a breakup, and it's brilliant. Ricardo Arjona and Gaby Moreno sing back and forth, pointing fingers and trying to decide whose fault it all was. The title itself, "Fuiste tú" [It was you], is the whole point of the song. He starts by saying she was inconsistent and sad, "Lo tuyo fue la intermitencia y la melancolía" [Yours was intermittence and melancholy], while he just accepted everything because he loved her. He throws out some fantastic, classic Arjona lines about how love isn't perfect, like "O aprendes a querer la espina o no aceptes rosas" [Either you learn to love the thorn or don't accept roses]. Basically, he's saying she couldn't handle the tough parts.
But then Gaby Moreno gives her side, and it's just as convincing. She feels exhausted and lost, singing that she's been seen "caminar descalza por la madrugada" [walking barefoot in the early morning]. Her big point is that he never accepted her for who she was. She sings, "Estoy en medio del que soy y del que tú quisieras" [I'm in the middle of who I am and who you'd want me to be]. They both agree on one thing: the beginning was easy. They both sing, "Que fácil fue tocar el cielo la primera vez" [How easy it was to touch the sky the first time]. But in the end, they just go back to blaming each other, repeating "Fuiste tú" [It was you] until the very end.