If you gain life at the same time a creature you control is dealt lethal damage, that creature dies before Lisette’s ability can put a +1/+1 counter on it.

If a nontoken creature an opponent controls would die, and both Valentin and another effect are trying to exile it instead, that opponent chooses which effect to apply. If Valentin’s effect ends up not applying because of this choice, the triggered ability that allows you to pay won’t trigger, even if the creature card ends up in exile.

If you gain an amount of life “for each” of something, that life is gained as one event and the ability will trigger only once.

You may only pay once for each life gain event.

An ability that triggers “whenever you gain life” triggers just once for each life-gaining event, no matter how much life you gain.

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, a “whenever you gain life” ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.