Basically any encounter with Destroyers is fine, just drop the concoction in their vicinity and the quest will update as completed. Here are some repeatable quests that guarantee you an encounter with Destroyers immediately and without much walking: * Go to Olaf Olafson, accept the "A Gate Too Far" quest. * Take the G.O.L.E.M. quest and use on the Destroyers while approaching the golem kilns. * During the final battle with the Great Destroyer. * Doing Glint's Challenge.