This is a simple guide that will include a description of where you can find each of the Coats of Arms (CoA) collectibles to unlock all the weapon skins. They're usually pretty easy to find once you know where to look, so you should be fine with text descriptions of the places. Once you collect a CoA you can safely exit the level and you'll still have it saved.

Each difficulty has two spoilers:
• The short one in the brackets acts as a hint, letting you know what area the CoA is in. For example "1.0" would mean it's in the first fighting arena, while "3.5" would mean it's in the transition area between arena 3 and 4, but not necessarily in the middle of it.
• The long text afterwards gives you instructions on how to get to the exact location. Or at least best I can do via text. Images are surprisingly not useful, especially since the collectibles don't reappear after you collected them.

• Lamb — {3.0} Right after you enter the 3rd arena turn right. It's behind the exploding crystal in that room
  
• Goat — {4.0} Above the pit of lava in 4th arena. Easily visible when you first enter
  
• Beast — {2.5} After leaving the 2nd arena and entering the crypts, go to the back right corner. The CoA is underneath the ceiling in the middle of the room
  
• Archdevil — {0.5} Above where the first enemy spawns

• Lamb — {4.0} In the 4th arena, just before the exit, on left of it
  
• Goat — {3.5} After you leave the 3rd arena, it will be far right of the main path to the next one. You can find it on some floating containers
  
• Beast — {1.5} Right above the exit of the first arena
  
• Archdevil — {2.0} After reaching the 2nd arena hug the cliff. In the very corner of the arena, the CoA will be hidden below the edge of the floor

• Lamb — {4.0} In the 4th arena, under the bridge in its center
  
• Goat — {3.5} At the top of the large staircase after the 3rd arena. It will be hovering in the center of the empty space the staircase circles around
  
• Beast — {3.0} It's in the corner of 3rd arena, underneath the ceiling. Go opposite wall of the exit, and from the top of the staircase look slightly left (and up)
  
• Archdevil — {1.0} In the 1st arena, on the outside of the wall on the right of the entrance

• Lamb — {3.0} After dropping into the 3rd arena hug the right wall. The CoA is in a small alcove right before the lava pool on the edge of the room
  
• Goat — {4.5} Right before the bossfight, instead of turning right towards the staircase to the boss arena, turn left. The CoA is on top of some rubble in a side cavern.
  
• Beast — {1.5} Beyond the first wave of enemies after the first arena, do a sharp right turn after passing a stone gate. CoA is on a small ledge above the lava moat
  
• Archdevil — {3.5} Right before the 4th arena, behind the last rock pillar on the left, high up

• Lamb — {3.5} After leaving the 3rd arena take the right path across the bridge. CoA will be near the edge before you go through the door at the end
  
• Goat — {2.5} In the first room after leaving the 2nd arena (one with a staircase and enemies), it will be above the entrance door
  
• Beast — {4.5} After leaving the underground of 4th arena, the CoA will be up high behind the first arch
  
• Archdevil — {1.5} After the 1st arena and before the seraph spawn, it'll be down the cliff on the left

• Lamb — {2.5} Just after the void stalker spawn after the 2nd arena. Go forward until the edge, then drop down and turn around
  
• Goat — {3.5} Right after exiting the 3rd arena, it'll be up high between two pillars
  
• Beast — {4.5} Above the exit of the 4th arena (on the outside)
  
• Archdevil — {1.5} After the 1st arena, but before you fall down the hole. Look up high in the back of the ruins

• Lamb — {3.5} Before the 4th arena, after you first drop into the area filled with containers. It'll be on the left of the entrance, on containers, not ground level
  
• Goat — {2.5} Shortly after the 2nd arena, on the left side near the ceiling
  
• Beast — {3.0} Back left corner of the 3rd arena, up high
  
• Archdevil — {1.5} Turn left after the 1st arena, and right before the path turns right turn around instead. One of the containers has their back open, and inside is the CoA

• Lamb — {0.5} After reaching the bridge at the very start of the game turn around and it will be in the mouth of a statue
  
• Goat — {1.5} In the middle of all the 'arches' right after the first arena (in the air)
  
• Beast — {2.5} Right before entering the third arena, look up
  
• Archdevil — {3.5} Right before entering the fourth arena, turn around and look down the cliff. There is a hidden alcove with the CoA peeking out

MuStudios created a video tutorial using the guide, so it's only fair to add it in here! IMPORTANT NOTE: Archdevil Stygia showcases wrong corner of the arena, but the general gist of the location is still right. Everything else should hopefully be of great help if my text descriptions are not enough.