All of your units attack simultaneously (you can also choose not to attack with some of them). So two level 1 units that have one attack upgrade each, can kill a level 2 unit. Or a level 3 unit that has two attack upgrades (remember, level 3 units and the crabs are the only ones that can have more than one attack upgrade at any one time), can attack and kill one level 1 unit and one influence marker. It`s up to the player.