Desarmas comes from the verb desarmar, meaning "to disarm." While it literally means to take away weapons, it's often used metaphorically to describe someone who makes you feel vulnerable or powerless in a charming way.
In this song, the singer uses it to express how his lover's gaze completely captivates him, making him feel utterly defenseless and smitten. It's a vivid and romantic way to describe the power of love.
"Solamente Tú" is a love song built almost entirely on one idea repeated from different angles: whatever else happens, it's still you. Cali Y El Dandee open with "Con tu mirada me desarmas como me desarmas tú, lo sé" ("With your gaze you disarm me as you disarm me, I know"), then spend the rest of the song describing that disarming as something closer to captivity than romance — "Solo con amor tú me atrapaste y siempre estaré preso" ("Only with love did you catch me and I'll always be in jail"). The chorus keeps circling back to the same certainty, "Para siempre serás tú" ("Forever it will be you"), which is what gives the song its title. It's simple, repetitive Spanish by design — verses and choruses share a lot of the same vocabulary — which makes it an easy song to actually follow line by line rather than just enjoy in the background.