The Rav-Ages of Caustic-X in Dead Island 2 is obtained automatically after completing Michael Anders and the Holy Grail story quest. For this quest, head to the Halperin Hotel to rescue Rav. First check in the hotel’s ballroom. You’ll find him up on the balcony. When ready, speak with him to learn that he needs data. To find the ambulance, exit the hotel then travel to the eastern side of the parking lot.
It will be guarded by some zombies so take them out then pick up the Caustic-X Shipment Receipt on a crate near the ambulance.
Return to the ballroom and crank up the tunes on the stage. This of course draws a crowd of zombies so take them all out. Rav will check in via radio afterwards and you’ll then need to head to the Halperin swimming pool to melt some zombies. Once there, kick 14 into the pool then return triumphantly back inside the hotel. In the ballroom, speak with Rav on the balcony to complete the quest. As a reward, you’ll receive the Rare Melee Liquidator Mod and the Caustic-X Bomb.