First you must buy the formula for the Dragon's Dream Bomb from the Herbalist in the east of Novigrad. He also has the required ingredients in stock. Buy them and craft the bomb. Now look for a group of monsters or humans without armor. Use Igni on them to make them burn, then immediately throw the Dragon's Dream Bomb on the burning enemy. This will ignite the gas produced by the bomb and cause an explosion. You must repeat this 10 times for the trophy / achievement to unlock.

If you have strong alcohol in your inventory you can meditate to replenish your bombs. Fast travel to another map (e.g. Royal Palace of Vizima) to make enemies respawn, or do it against different enemies every now and then if you don't want to farm it in one spot (can also be combined nicely with destroying monster nests).