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