Another suggestion: You can go with the "single big creature" approach. While your opponent is sending out tons of weeny creatures, you summon a big bad creature and then use the rest of your mana to make him virtually unkillable (nullify, block, reverse attack, reverse magic, bear strength, bull endurance, cheetah speed, etc.) then he should be taking out the little creatures one at a time pretty easily. While your opponent is running out of creatures to send at you, your one creature is just destroying them.
Another option is that the best defense is a good offense. Sometimes against a swarm, I'll just totally ignore the swarm and start throwing fireballs at the enemy mage, throw my big creature at him, or attack him with a mage with tons of equipment. In this way, he now has to focus on defense, but generally if a player has put all they've got into swarming, they don't have any walls, or mana crystals, or equipment on them.