Start a (Solo) Game with the Chapter "Labyrinth of Death" from Episode 1 and set the Enemy Spawn to "For 4 Players". Choose 2 Dynamite and 2 Stiehlhandgranate as next. In the Chapter, go forward and put the Dynamite on the Ground, activate all enemies and gather all at the Dynamite Place. Stick all Zombies together, throw 2 Grenades in the pulk of Zombies. You will earn all 3 explosion Achievements at once with this tactics.