This achievement can be easily unlocked in the tutorial world but note that the layout of the tutorial world sometimes changes but the method to unlocking is still the same.
Load up the tutorial and complete the basic features so you can go through the opening in the wall.
Once you have gone through the wall you're in a town, pass the water fountain and keep going till you come to an area with potions bottles in frames by it, the area has a cauldron, chest and brewing stand.
Firstly grab everything from the chests then with the glass bottles equipped press
at the cauldrons to turn them into water bottles (3). Now press
onto the brewing stand and place your 3 water bottles into the stand and place Nether Wart at the top and let it brew.
You should now have 3 Awkward potions, now place Magma Cream at the top and let it brew again which will give you your Fire Resistance potions. Grab them and head over to the room with the anvil opposite.
Drink the potion and break the cobblestone in front of the lava and swim!
Here's a video (The potion area may be different but the method is the same)