For each ship you kill you get 30 merits. Worth pointing out given the other answer, you undermine in enemy systems.
If you are in a wing and you all hit the enemy you all get 30 merits, not a share. So it is really worth winging up to undermine as it speeds things up no end.
Check the Huddson Reddit for the group target for undermining, if we pull in the same direction we can achieve more.