Here is the full list of all 15 Guards achievements.
Learn all skills
Defeat the chieftain at hell difficulty
Close the portal with default heroes
Kill 5 enemies using one hero in one turn
Terminate 300 enemies in one session
Kill the chieftain at nightmare difficulty
Open all items in item shop
Save up 777 mifril
Find and kill the Hell Boar
Deal 250 damage in one turn
Defeat the chieftain!
Liquidate 200 enemies in one session
Hire all guards
Destroy 100 enemies in one session
Kill 3 enemies using one hero in one turn