Grieve

Get more than 50 headshots

Take no damage in level 3

Perform more than 10 takedowns (Takedowns are performed by quietly walking behind enemies and taking them out)

Respawn from check points more than 10 times after dying.

loot all ammo crates in a level

Eliminate 10 enemies with a grenade

Do not take damage when you complete a level on hardcore difficulty

Complete level 2 or level 3 or level 4 without respawning from death once

Finish level 2 or level 3 or level 4 without using a medkit

Finish the whole game on any difficulty.