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