You will earn this skill in level 1 of the campaign. When you attack enemies, your hit counter and combo gauge in the bottom-right corner will go up. When it fills up, the number in the middle will increase.
You will need at least 1 in your combo gauge to perform a ruiner attack. This is done by pressing , which will let out an explosion and damage/kill anyone caught in it.
NOTE: Playing in co-op if player 1 kills 10 enemies with a ruiner attack, both player 1 and 2 will receive 10 kills and visa-versa