Heliod, Sun-Crowned must be on the battlefield. Walking Ballista must be on the battlefield. Walking Ballista has at least two +1/+1 counters on it. Mana needed:
Steps
Activate Heliod by paying , giving Walking Ballista lifelink until end of turn.
Activate Walking Ballista's second ability by removing a +1/+1 counter, dealing 1 damage to any target.
Walking Ballista's lifelink triggers, causing you to gain 1 life.
Heliod triggers, putting a +1/+! counter on Walking Ballista.