Here is the full list of all 20 Enemy achievements.
Clear an area with just one team member left alive.
Deal over 75 points of damage in a single attack.
Kill an enemy with gradual fire damage over time or from stepping on fire.
Cause an enemy to panic.
An enemy dies from collision damage.
Reach a boss castle.
Defeat all the enemies in a town attack.
Save 50% of the civilians in a town attack.
Save 75% of the civilians in a town attack.
Recruit a full squad.
Deal over 125 points of damage in a single attack.
Make it through the night.
Defeat the Glob.
Defeat the Machine Man.
Defeat the Cyclops.
Defeat the Count.
Defeat a boss.
Defeat two bosses on a single playthrough.
Defeat three bosses on a single playthrough.
Defeat all four bosses and win the game.