Kill 500 Skeletons

Get an unbroken 10-headshot streak

Reach a Chain Count of 30

Kill 20 enemies as they revive

Defeat 500 enemies with headshots

Kill a Suicide Zombie by hitting his grenade

Sever 1,000 limbs

Enchant or upgrade a piece of equipment in single-player mode.

Acquire the Bow of Apollo

Rip apart 1 Olympus Sentry

Discover a veilfire rune.

Clear the "Ground Zeroes" mission