Fast travel to another map to make the bandits respawn. You can meditate to replenish your bombs. The three effects must be active on one enemy at the same time.

Stay very close to enemies so that they can't run away from the poison cloud. Start by shooting the broadhead bolt, followed by the Devil's Puffball and then the Dancing Star Bomb.

Be sure to set the difficulty to Death March, otherwise they will die too quickly. Once you have all these things head to the Devil's Pit Bandit Camp in Velen. The two bomb formulae can be bought at the Herbalist's Hut in Novigrad. The Dancing Star Bomb does burning damage and has a much higher chance of burning enemies than Igni (but it's possible with Igni). The Devil's Puffball creates a poison cloud and the enemy must be standing inside the cloud to receive poison damage. The Broadhead Bolts are available at most blacksmiths and have a 100% bleeding chance (each shot will always make enemies bleed for 15 seconds). A sure shot way to get this trophy / achievement is to use Broadhead Bolts (bleeding), Devil's Puffball Bombs (poisoning) and Dancing Star Bombs (burning).

Witcher 3 overkill achivement