Mortal Shell - All Shell Locations

I would suggest finding all the shells and weapons as you enter the Fallgrim Tower and the beginning of the game. This way you will get them all right at the start and you can choose whatever you see fit for the rest of the game. It's also easier to stick with one shell and weapon and to upgrade just those. This video will show you all the locations and even the boss fights to get the weapons, so be cure to check the description for timestamps to easily skip parts you maybe don't need.

0:04 - Harros Shell - When you wake up in Fallgrim, after the tunel take the first left and pick it up
0:29 - Eredrim Shell - From the Fallgrim Tower go towards the Seat of Infinity, just before the bridge leading there look on the left
2:16 - Solomon Shell - From the Fallgrim Tower go straigth from the lower floor door and to the left through the hole
3:53 - Tiel Shell - On the path which leads to the Crypt of the Martyr, there is a hole on the right side
6:03 - Hammer and Chisel Weapon - Defeat boss at the beginning of the Seat of Infinity area
8:15 - Boss fight
11:56 - Smoldering Mace Weapon - Defeat boss at the beginning of the Shrine of Ash area
14:50 - Boss fight
18:18 - Martyr's Blade Weapon - Defeat boss at the beginning of the Crypt of the Martyrs area
20:16 - Boss fight