Crocodile got messed up by bloody fists. Maybe not the best example of Logia invulnerability. Lol. Anyway, Logia can reform from being splattered but they can't recover from their element itself being destroyed or having a direct counter to their element. That's the very reason Ace lost. "The power to burn even fire". Akainu's elemental superiority defeated Ace. It's also why Crocodile was beaten because sand doesn't disperse well when wet. Magma itself has an evaporation point. As does sand or ice. An intense enough explosion would kill them. Kizaru and Enel however are exempt because they are pure energy.