Arremangando comes from the verb arremangar, which literally means "to roll up one's sleeves." While it can refer to physically rolling up sleeves, in this context, it's a powerful idiom.
It signifies getting ready for action, preparing for a confrontation, or showing a readiness to work or fight. The lyric "vengo arremangando" implies the speaker is coming prepared and ready to deal with any trouble, especially those who are "de listo" (acting smart) or "robando" (stealing).
Virlan Garcia's "Flaco" is a powerful track where he reflects on his journey and rise to power. He starts by saying he never imagined he'd go from fighting to being a "school" himself, meaning he's become a master of his craft. He talks about how many people now want to join his "boat" that he "capitalized," built from his "hunger for power here on the border."
He describes himself as always ready for war, equipped with his people and a tactical team. He mentions the Arellano organization, which opened the door for him to the world of the mafia, where he found his profession and started making money. He enjoys the action, driving sports cars, and even shooting guns into the wind. He also makes it clear that he's a man of his word, loyal to his people and family, and commands respect. When he's in Tijuana, he's with his team patrolling, and he mentions his nickname, "El Flaco," and how he can quickly resolve any conflict. He's always on guard, ready to deal with anyone who tries to be smart or steal, warning that "the earth will swallow them" because he's "very angry."