All charged up 25G
Killed 5 enemies with one Arc Cannon shot

"The Arc Cannon is an immensely powerful gun, firing a lightning bolt at a low rate of fire. It can be fired quickly and semiautomatically, or charged up for high power. When firing the arc cannon at a cluster of enemies, plasma arcs will erupt from the main point of contact, connecting enemies with the target, and electrocuting the group."

Any type of enemy will count for this achievement, so find a group of AI soldiers and charge a single shot, aim for the middle, and fire away. The Arc Cannon is unlocked at Pilot Level 21. The Capacitor mod (unlocked by killing 20 Titans with this weapon) will help with this achievement. It also seems you can get the achievement by simply spamming the weapon and killing five in quick succession. It may be that the counter starts when the first chain goes off, and as long as you keep a chain going you will be able to get it.