This achievement is gained by entering lava while you have a fire resistance effect active. This must be done in survival mode. Most of this can be done on any difficulty, but some parts may require monsters, and thus exempts peaceful for that time.
To get a fire resistance effect you can drink a potion of fire resistance. This potion requires some normal and Nether raw materials; 3x sand, body of water, potion stand, nether wart, and magma cream.
Nether wart can be found growing on soul sand in nether fortresses, next to stairs. Magma cream can be crafted with slime and blaze powder, or you can collect it as a Magma Cube drop when they die.Create Potion:
1. Smelt the sand to create glass. Use the glass to create a bottle.
2. Use the bottle on the body of water (
or right click).
3. Place the bottle of water and nether wart in the potion stand.
4. Once the awkward potion has finished brewing, place it and the magma cream in the potion stand.
5. (Optional) Now you have a fire resistance potion, and can put it back in with redstone to make a stronger potion.
Now go and find some lava, drink the potion, then jump in.