The infusion ability will trigger when Poisoner's Apprentice enters regardless of whether you've gained life this turn, and you'll choose a target. As that ability resolves, it will check to see if you've gained life this turn. If you have, the target creature will get -4/-4 until end of turn.
Infusion — When this creature enters, target creature an opponent controls gets -4/-4 until end of turn if you gained life this turn.
