This can be done with any character but is probably easiest to achieve with the archer, and best done on the Butcher's level. Use the card that gives you multiple arrows and upgrade it all the way. As you go through this level, you will see many barrels. When you destroy them they will either explade, create monsters (mostly skeletons and spiders), or create more barrels. It is mostly luck, but what you are hoping for is a large cluster of barrels. When you explode one, it may set off a chain reaction, blowing up everything nearby, those creating more monsters or barrels or also exlpoding, keeping the chain reaction going. Just be sure to keep shooting so that the monsters are dieing and barrels exploding as quickly as they are created. With luck, you will reach the required limit quickly. As I said though, it's mostly luck so you may need to spend a few keys before it happens. The first level of difficulty is the best since the monsters will have only a few hearts and die easily, assuming you are high enough level. If the monsters aren't dieing almost immediately, level up some more and come back later.