For boosting purposes grab a friend, open up a private skirmish death match, put resources on to 9999 and put on any positive skulls. Make sure you choose Kinsano, whilst your friend opts for a Banished leader. Build an armoury, upgrade kinsano's mech.
Get your friend to mass produce grunts and have them sent to an open area. You might want to build some nightingales to heal Kinsano, but it's up to you.
Once the grunts are stockpiled in this one location, and Kinsano is close enough press
and direct the firewall at them. You should get this achievement within 1-3 goes. P.S. there is a cool down rate on the firewall ability, so you might want to retreat until you can slaughter the grunts again.
Switch roles and both of you should have the achievement. Happy hunting.