Your first encounter with Dark Samus occurs in the Temple Assembly Site. This battle is more of a chase sequence and introduction to her abilities rather than a full-fledged boss fight. Dark Samus will primarily use her Phazon Beam and Phazon Missiles. She is incredibly agile and difficult to hit consistently.
Strategy: The goal here is not to defeat her, but to survive and drive her off. Focus on dodging her attacks. Use your Power Beam and Dark Beam to deal damage when you have an opening, but don't overcommit. She will frequently disappear and reappear. After taking a certain amount of damage, she will retreat. This encounter serves to establish her as a formidable recurring antagonist. There is no specific reward for this encounter other than story progression.