When you kill an enemy before the combo bar depletes, your combo increases by one. The bigger the combo, the less time you have before it breaks. Your combo will multiply your score AND your energy.
Energy will allow you to use a dash attack. When you hold left click, charge an attack and launch yourself towards your cursor. This attack pierces enemies.
Enemies will always aim for the core. Killing them will give you an amount of energy depending on your current combo, but it will also slightly increase the speed of every other enemy.
The core :
The core has 4 health point represented by how chipped up it is. The game stops when it reaches 0 so you have to do your best to protect it. When hit by an enemy, the core will change trajectory for a moment, which can make it bounce on other enemies and be extremely dangerous.