Samurai mode - Successfully complete the game to unlock the Samurai difficulty.

Tourney mode - Successfully complete the game on the Samurai difficulty to unlock Tourney mode.

Black shirt - Successfully complete the game to unlock the black shirt.

Samurai hat - Successfully complete the game on the Samurai difficulty to unlock the Samurai hat.

Secret Gamerpic - Successfully complete the game on the Samurai difficulty to unlock the Samurai Dishwasher Gamerpic.

Guitar solo locations - You can do guitar solos at the following locations:

Drome Rail: Found while proceeding through the level.
L'opera Dramatique: Kill the chainsaw mini-boss. Kill all the zombies in the next two rooms, and the guitar will appear.
In Processing: Found while proceeding through the level.

Pick locations - Picks can be found at the following locations:

Foghorn Cafe - Clear the second room, then go all the way back to the room before the one that you started at. Successfully complete the time trial to get the pick.

Ivy Trail - Reach the area where you can either go up or right. Continue going to the right until you reach the time trial. Successfully complete the time trial to get the pick.

Gray Fortress - Clear the large room with the jet-pack cyborgs, then enter the room on the right side of that area. The pick is inside a container.

St. Crux Cemetery - Go to the right from the room where you found the cemetery key. Pass the vendor bot, then go up using the shift blade until you find the killer solo amp.

Nord Trainyard - Successfully complete the time trial when you first enter the warehouse to get the pick.

Drome Rail - Get the crusher dish magic, and enter the next room. Successfully complete the time trial to get the pick.

On the Samurai difficulty, you will become caged in and attacked on your way to the last cart. Kill the enemies, and the killer solo amp will appear.

L'Opera Dramatique - Clear the enemies at theater entrance. The killer solo amp will appear above the entrance.

From the last killer amp, use your shift blade, and go up to find an opening in ceiling. The pick is in the container.

On the Samurai difficulty, defeat the chainsaw boss, then clear the zombie infested room. Break the container that drops down, and the killer solo amp will appear.

Metatron District - Enter the room after you meet the chef. Successfully complete the time trial to get the pick.

Grace Chapel - Clear the room before the locked chapel door.

Mono Catacombs - Clear the room full of zombies.

Cam Rooftops - Clear the third room.

A killer solo amp will appear before the last room.

Cam Lobby - While falling, move to the left wall. When dropping to the next room you should land on a platform with a container. Break open the container to get the pick.

In Processing - Successfully complete the time trial after the vendor bot to get the pick.

On the Samurai difficulty, the killer solo amp will appear in a room on the way to get the two electro-spheres.

Cam Core - Use the shift blade, and go straight up in the first room to enter a room with a container. Break the container to get the pick.

Amulet Of Phantom Rock location - Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Smasher of Zombies (25 points): Beat the game on normal or ninja difficulty.
The Peter Moore Achievement (8 points): Pay homage to one of our generation's greatest rock performances ever.
Got the Hat (40 points): Beat the game on Samurai difficulty.
The Black Book Achievement (11 points): Earn 100 million points in an Arcade level.
Cemetery Man (20 points): Complete St. Crux Cemetery using only the Chainsaw.
The Brutal Achievement (10 points): Do 10 wall slams in one combo.
Kill You Seven Times (14 points): Air juggle an enemy for 10 seconds.
Untouchable (21 points): Get 21 kills without getting hit once.
Master of Arcade Dishwashing (15 points): Complete Arcade mode.
And You Will Know Us... (13 points): Impale 13 freaks.

Additionally, there are two secret achievements:

The First Encounter (10 points): Unknown.
Smash Your TV (13 points): Unknown.