You're probably asking too broad a question, really.

Advent can get away with a decent number of overall strategies by playing on their unique fleet synergies. They overall tend to have less resources to work with, but pound for pound, ship for ship, Advent fleets later in game are probably the most dangerous.

If you could narrow your inquiry it would probably help greatly, like if you're asking about how Advent can defend, how they can attack, fleet composition, their economy, etc..