On this page of the trophy guide to Resident Evil Village , you will learn how to get the Medium Rare achievement. It is awarded for setting Moroaica (one of regular monsters) on fire in the story mode.
Moroaica is a monster variety, the one with a hood and a sword. They can be found in fixed places in the game world.
A good opportunity to get the trophy comes when you return to the village from the Reservoir area, which is after defeating the boss Moreau. Ethan will acquire a crank in the swamps.
Reach the Drawbridge shown in the pictures - it is located west of the Altar (the one where Duke is). You can use the crank to lower the drawbridge.
By lowering the bridge, you will reach the boat. You can get on it and you have to sail it north.
After reaching the new boat mooring, head east towards the passage marked on the picture.
You will reach a small cell with a brazier lit and can notice two side rooms that are inaccessible. You can unlock access to these rooms by firing at the burning brazier and pushing it towards the other inactive ones to light them up as well.
A monster will appear in one of the side rooms. Let creature approach Ethan, but stay near the ignited brazier. As Moroaica follows the protagonist, you need to try and make it accidentally ignite itself.