2nd boss fight.
Ibn is the healer who keeps healing Bax for 10 HP and Bax is the attacker who keeps using his "special" attack on you.
Focus on Ibn, who hangs back at the left side. Two perfectly executed Ignite should finish him off. After that go all out on Bax until he's down to pop the achievement.
To make things easier, you can change the settings to allow the game to automatically give you a "great" on the QTEs for both attack and defense so that you always get the most damage when attacking and suffer the least damage when defending. This doesn't affect achievements.
OR if you really don't feel like fighting, you can turn "Instant Victory" on in settings. This allows you to choose "Victory" as an option when fighting to skip the fight altogether. This works for any fight, including boss encounters and DO NOT void achievements.