Dark swarm is used correctly will beat mass guardian strategy.
This spell is one of the best spells to negate any defense that protects a base. It will make any unit underneath it invincible to range attacks. To use this spell effectively, you must have a combination of zerglings and hydralisks. Hydralisks will be invincible from air units as guardians, battlecruisers, or carriers. Zerglings will be invincible from most defensive units and structures.
To use dark swarm you need a defiler, and you should research consume before producing defilers. You want to have a clear path to your opponents base, so scout ahead an burrow zerglings. Any upgrades would be helpful, as speed, attack, carapace, or even adrenal glands. Just keep consuming zerglings whenever you need to cast more dark swarms, which you should do constantly since dark swarm doesn't cover a huge area.
Dark swarm can be used defensively, and can be acquired before guardians. Just cast it on top of your hydralisks, and any air siege units will not be able to harm them. However your opponent can just retreat and return when it wears off. Having plague will hurt your opponent, since he won't be able to kill the defiler with just range attacks.
An interesting use for dark swarm is to hide the amount of units that you have, since multiple darkswarms on top of each other will make it impossible for your anyone to see underneath.
