Here is the full list of all 19 Tentacult achievements.
Complete Level 3
Complete Level 2
Complete Level 1
Transport Your First Friend
Convert Your First Friend
Convert 1000 Women
Convert 1000 Men
Convert 100 Snipers
Convert 100 Grenadiers
Transport 10,000 Friends to R'lyeh
Lose 10,000 Friends to the Enemy
Kill 100 Friends Using Cars
Beat a Level with 80 Friends or More
Complete Level 4
Complete Level 5
Complete Level 6
Complete Level 7
Complete Level 8
Convert 100 Old Folks