If Overgrowth Elemental dies at the same time as one or more other creatures you control, Overgrowth Elemental’s last ability triggers for each of those other creatures. You will gain 1 life for each of them, but if any of those creatures were Elementals, Overgrowth Elemental won’t get a +1/+1 counter since it’s already left the battlefield.

If a creature you control dies at the same time that your life total becomes 0 or less, you lose the game before Overgrowth Elemental’s last ability can save you.