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