What's a word/phrase for a player/seducer?

You might also be looking for the phrase pickup artist.

This used to be a synonym for a "ladies' man," someone who was good at the skills and techniques of seduction.

However, it's recently begun to be associated with a specific subset of extremely creepy people who use a specific range of extremely creepy techniques to pick up women. If you are talking about people taking things "seriously," this might be what you mean--but you should be aware that most people now consider "pickup artist" to also carry the strong implication of being highly misogynistic and a borderline psychopath.

"Smooth operator" is someone who is good at winning over women. It's also the title of a song about that same behavior - it has a slightly negative connotation.

It might be exciting to get the attention of a smooth operator (or if you're a man, to have the skills of one) but you probably wouldn't want to take him home to meet your parents.

How about smooth-talker

Definition in Urban dictionary:

Someone who has a very rare ability to say all the right things at the right time. Even when a situation is not in their favor, by the end of the night their uncanny ability with words has turned a unfavorable situation into an advantage. They are charming and possess all the people skills then some, they can talk their way out of or in into anything they want. Even more deadly if the person is hot.

Incubus is a paranormal term for a male entity that seduces women. A demon with supernatural abilities.

Ace is a skilled performer in many contexts.

Gigolo is another word for a man who is a professional romancer.

Rock Star is one who attracts women, but doesn't necessarily have skill. See also movie star.

Perhaps:

He is a real ladies man.