Here is the full list of all 18 Hex Slayer achievements.
Complete the "Land Grab" campaign
Complete the "Archers Away" campaign
Complete the "Hectic Hoards" campaign
Complete the "Magic Time" campaign
Complete the "Assassination" campaign
Dominate an island in Battle Mode
Dominate an island at Normal difficulty in battle mode
Dominate an island at Tough difficulty in battle mode
Dominate an island at Hard difficulty in battle mode
Dominate an island at Challenging difficulty in battle mode
Dominate an island at Extreme difficulty in battle mode
Dominate an island at Absurd difficulty in battle mode
Dominate an island at Impossible difficulty in battle mode
Score 10,000 points in endless
Score 20,000 points in endless
Score 30,000 points in endless
Score 40,000 points in endless
Score 50,000 points in endless