Quest for Pure Flame has at least four quest counters on it.
You have a way to give
Walking Ballista lifelink.
Walking Ballista has at least two +1/+1 counters on it, or your life total is at least 4.
Steps
Activate
Quest for Pure Flame by removing four quest counters from it and sacrificing it, doubling all damage dealt by sources you control this turn.
Give
Walking Ballista lifelink, if needed.
If
Walking Ballista has less than two +1/+1 counters on it, activate
Unspeakable Symbol by paying 3 life, putting a +1/+1 counter on
Walking Ballista.
Activate
Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 4 damage to an opponent or a permanent an opponent controls, and causing you to gain 4 life.