How do you get skulls in Orcs Must Die 2?
To expand on mordi2k's answer a bit:
Basic map skulls can be earned only once, and are earned 2 per map in Apprentice and 5 per map in War Mage difficulty. These skulls are shared between difficulties, earning them in one of the difficulties automatically gives them on the other one.
At the same time, repeating a map in Nightmare difficulty (unlocked after completing the game) allows players to earn 5 more skulls per map that are not shared with the lower difficulties.
Of the 5 skulls earnable in the maps, 4 are based on how many Rift Points are left when the level is complete, and the remaining one is earned if the level is complete within the level's par time.
Bonus skulls
Here is the list of bonus skulls you can earn at the end of a level:
- Victory, 1 skull: Win a level. (2 skulls on Nightmare)
- Perfect Victory, 1 skull: Don't lose any Rift Point. (2 skulls on Nightmare)
- Perfect Buddy Victory, 1 skull: Don't lose any Rift Point in co-op game. (? skulls on Nightmare)
- Untouchable, 2 skulls: Never be hit. Vampiric Gauntlets don't count against this bonus. (5 skulls on Nightmare)
- Trapless, 3 skulls: Don't use any Traps or Guardians. (6 skulls on Nightmare)
- Manaless, 3 skulls: Don't use any mana. (6 skulls on Nightmare)
- New High Killstreak, 2 skulls: Make a killstreak higher than any precedent on that character. (5 skulls on Nightmare)
- New High Score, 1 skull: Beat your preceding scores on an already played map. (2 skulls on Nightmare)
- Skulls Picked up, 1 skull each: 1 for each skull picked up during the mission (dropped by enemies), it is not shared with the other player. (Question on what causes skulls to drop)
- 100 Career Kills, 1 skull: Kill 100 enemies.
- 200 Career Kills, 1 skull: Kill 200 enemies.
- 300 Career Kills, 1 skull: Kill 300 enemies.
- 400 Career Kills, 1 skull: Kill 400 enemies.
- 500 Career Kills, 1 skull: Kill 500 enemies.
- 1000 Career Kills, 1 skull: Kill 1000 enemies.
- 5000 Career Kills, 5 skulls: Kill 5000 enemies.
- 10000 Career Kills, 5 skulls: Kill 10000 enemies.
- 20000 Career Kills, 5 skulls: Kill 20000 enemies.
- 30000 Career Kills, 5 skulls: Kill 30000 enemies.
- 40000 Career Kills, 5 skulls: Kill 40000 enemies.
- 50000 Career Kills, 5 skulls: Kill 50000 enemies.
- 100000 Career Kills, 10 skulls: Kill 100000 enemies.
- 500 More Career Kills, 1 skull: Kill 500 more enemies. Earned only when the others Career Kills milestones aren't met.
- 1000 More Career Kills, 2 skulls: Kill 1000 more enemies. Earned only when the others - Career Kills milestones aren't met.
All information obtained from the Orcs Must Die Wikia
If you look at the stats page after the level is done, you'll see that you get;
- Max 4 skulls from not losing any rift points. (You will lose rift points by either dying, or letting enemies get through the rift.)
- 1 skull for completing the level within par time, which varies depending on the level.
You can also gain extra skulls from reaching certain milestones, or getting them as loot from enemies (note that these skulls do not count against the 5 skull scoring system).