Cobardes means "cowards". It's a very strong word, and in this song, it's used in a surprisingly desperate and unique way.
The singer begs her ex to "Haz las cosas que hacen los cobardes" (Do the things that cowards do). She wants him to stop being kind and instead act cruelly, giving her a reason to finally hate him and move on from their painfully perfect past.
Have you ever run into an ex and wished they were just a total jerk instead of being sweet? That is the exact nightmare HaAsh sings about in "Ex De Verdad". The singer bumps into an old flame in a huge city, ruining her peaceful day. She locked away her feelings "con candado en un cajón" ("with a padlock in a drawer"), but one warm smile completely unlocks the box. He even greets her with a casual kiss ("un beso sin pensar"*), totally blind to how fragile she still feels. It drives her crazy because he is "tan perfecto que no te olvido" ("so perfect that I can't forget you").
Her solution is hilariously blunt: she begs him to ruin his own reputation. Instead of being kind, she pleads, "Sé un ex de verdad y trátame mal" ("Be a real ex and treat me badly"). In pop culture, a "real ex" is usually selfish, rude, or distant. She actually wants him to act cowardly ("Haz las cosas que hacen los cobardes" / "Do the things cowards do") so she can finally feel angry instead of heartbroken. As long as he keeps smiling and acting sweet, her soul keeps suffering, so she literally asks him for a favor: "ayúdame a odiarte" ("help me hate you").