To unlock this achievement, you must reach at least 20 Hot Weather Defence in your stats.
This can be done entirely through the use of one potion. The Weather Defence Potion provides +20 to Hot Weather Defence and +20 to Cold Weather Defence. To craft it you will need an alchemy kit, Water, Greasy Fern and Crystal Powder.
Alternatively, you can visit the Cabal Hermit in north eastern Chersonese and learn the passive skill ‘Weather Tolerance’ for 100 silver, granting +8 Hot and Cold Weather Defence. Combine this with armour which provides Hot Weather Defence and you will achieve +20 Hot Weather Defence with ease.