Here is the full list of all 28 Siege of Centauri achievements.
Destroy the Heart of the Phoenix.
Beat the campaign on normal difficulty
Beat Sedge on normal or higher
Beat Styx on Normal difficulty.
Destroy the Nest of the Queen.
Beat Caina on normal or higher
Beat Sord on Normal difficulty.
Beat Plutus on Normal difficulty.
Beat Dule on Normal difficulty.
Beat Plain of Fire on Normal difficulty.
Beat Malebolge on Normal difficulty.
Win with less than 50 HP left on your colony.
Upgrade a colony to level 3.
Beat Ciacco on Normal difficulty.
Beat Epicurus on Normal difficulty.
Beat Gaol on Normal difficulty.
Beat Thurlos on Normal difficulty.
Beat Ascent on Normal difficulty.
Beat Lacuna on Normal difficulty.
Beat Gallehault on Normal difficulty.
Beat Chariah on Normal difficulty.
Beat Nahant on Normal difficulty.
Beat Rul on Normal difficulty.
Beat Caluck on Normal difficulty.
Beat Malden on Normal difficulty.
Beat Holden on normal or higher
Beat Circosta on Normal difficulty.
Beat Limbo.