You will need to have unlocked the Lightning Ring, by reaching 5000 kills.
Once it is unlocked, choose to upgrade it three times in-game (when you level up).
The Lightning Ring strikes enemies at random, and upgrading it strikes more enemies for more damage.
A good tactic is to also upgrade the Duplicator (more projectiles = more lightning).
Having your character' main weapon upgraded by a couple of levels first and then adding other projectile weapons (Wands) or a close-quarter attack (Garlic) will make the Lightning Ring more effective and protect you while you upgrade it.