Double Tap

Strategy and Synergy
Something of a "win more" spell. The circumstances where this spell would actually work as intended are contrived - you need to get an enemy down to 240 or less health, have this spell ready, and spend 4 mana on it. If you manage to kill your target, you immediately recast the spell, and have very little time to line up your two extra shots.

Essentially has no bonus effect against bosses, and after a time loop, enemies will have several thousand HP, making Double Tap even more of a niche spell.

However, it may be useful for clearing obstacles, if you're very worried about rocks.

Upgrades like or  are good options.

Trivia
Rule #2: Double Tap is a personal rule of the character Columbus from the movie Zombieland. "In those moments where you're not really sure if the undead are really dead dead, don't get all stingy with your bullets. One more clean shot to the head."