Well I needed the same thing for the 1st lv of Myricals arc. What I did is pile a bunch of boxes or barrels or windows around a barrel set the barrels name to whatever (lets say bomb), put spawn at start to NO, and spwan life to 1 and set spwan after delay to YES. then make a trigger zone, set ifused to "bomb" and put the main to plrinzoneactivateused. this will make it so when you walk into the zone the berral thats behind the boxes will explode and the boxes will break and you will have an exploshion.
1.A suggestion is to put wood planks all around the berral and some boxes that have 0 life, these entity will fly everywere and add to the effect.
2. another suggestion is to have alot of exploshion noses around, and gun fire, and people begging for there lifes, ect...
3. suggeston number three play the Myricals arc demo to see good places to place you exploshions.