The IN-GAME achievement works no problem. I have ticked off both requirements for this achievement under Ascending Tide, General - Ascending Tide Scout worth 10 in-game points however the Xbox achievement is bugged and does not pop:
Coral Aerie vanquisher (10)
- Defeat Maligalig
- Defeat Sarydil
- Defeat Varallion
Shipwright's Regret vanquisher (10)
- Defeat Foreman Bradiggan
- Defeat Nazaray
- Defeat Captain Numirril