It is the problem of most new Teams playing league battles there is no defense and your team loses. So what to do?
It's easy have you need some training but it's easy.
You need a Support weapon and why? Mostly because when a teammate is about to die in a battle you could give him/her a Ink armor or even with a Ink storm.
Next there should be a aggressive weapon that makes the enemy trying to mostly get out of your way so your team is holding the objective longer. For an example a Blaster because it can shoot around corners that makes it almost impossible to hide from you, or a charger if your good with it.
A melee weapon with ninja squid combined this is like a advanced kind of aggressiv playing but be careful long ranged weapons are going to be your greatest weakness so put your ink resistance out and make the enemy fear you.
Last but not least a weapon that makes damage over walls because everybody can hide behind walls and can get to a nightmare if you don't notice them they flank your team and you and your team loses the lead stick grenades or toxic mist should make them weaker point sensors are recommended so your team will be alarmed.
Thats all of it
Hopefully this helped you with your beginn in league battles.