I started using the Custom E-Liter 3K recently, so here are a few tips.
Wear gear with Damage Up. Three main DUs can kill with about 3/4 of a charge, and even if you miss, the enemy may step in the ink you just shot, which reduces his/her health.
Snap your snipes--charge up facing a nearby wall or obstacle so the enemy can't see the charge line, then quickly aim and take them out.
Place all three Squid Beakons in somewhat hidden locations--and sometimes, you can catch a distracted enemy shooting at one and take advantage of the situation.
Save the Kraken for opportune/last resort situations. I often take out at least two enemies when I use it.
On some maps, rushing to take the center or more right at the start works out. On Port Mackerel, get on top of the enemy's forklift thing and camp with your team (hopefully) close behind.