There are really two methods to get breaker mace kills fast. You can either play arcade or farm this in The Hive in Escape mode.

Arcade

Getting your breaker mace kills in arcade is going to be a little bit more difficult than farming your kills in Escape mode. Before you can actually use the mace, you will need to build up eliminations (7) to purchase the weapon.

Then, you need to do your best to not get blown away while you roam the battlefield in hopes of blood and glory.

The breaker mace has two different swings, so make some time to get acquainted with both. The power swing is able to deal area to the surrounding area and in a straight line from where you’re aiming, while the basic swing swipes across the screen. Both attacks are one-hit-kills.

Escape (The Hive)

Head into a solo match and start on beginner. The first large room (red room) after the safe-room will spawn 1 of 2 bosses (from what I’ve seen), the Swarm or the Warden.

You want the Warden.

Grab the boomshot from a supply room before fighting him, and kill him with it. Once he’s been vanquished, pick up his mace, and save it for the evac site.

After the doors at evac begin to close, waves of enemies will pour in (good spot to farm Juvies). Use the mace relentlessly.