I'm playing the Heliod, Sun-Crowned & Walking Ballista combo where Heliod is able to give +1/+1 to Walking Ballista when I gain life:
Whenever you gain life, put a +1/+1 counter on target creature or enchantment you control.
If my Walking Ballista only has one +1/+1 counter and I use its ability to target an opponent player, does the lifelink and consequent +1/+1 counter from Heliod save the Ballista from dying?