Here is the full list of all 21 Orc Slayer achievements.
Survive Level 8
Slay the Necromancer
Survive Level 7
Slay 30 Orc Skeletons
Survive Level 6
Survive Level 5
Slay the general
Slay Big Bertha, the colossal hen
find the hidden area
Survive Level 4
Survive Level 3
Slay 200 Orcs in one level
Slay a Tank
Survive Level 2
Slay 100 orcs in one level
Slay an orc with a shot to the groin
Surive Level 1
Slay 3 Orcs with a Pigboom
Slay 50 Orcs in one level
Slay an Orc with a shot to the Head
Find the hidden area within the woods