(crossposted from rules-day tuesday, when the mod chat couldn't agree)

Amy controls Lost Leonin and Raptor Companion and is attacking Nissa. Nissa is at 2 life and 8 poison, but has Healing Salve in hand. Can Nissa survive?

This boils down to if 615.7 allows you to choose what *source order* applies, or choose *any damage* to prevent, of all selected. It can be further expanded to more creatures (though made increasingly corner-casey) where you need to prevent exactly 1 damage from each of 3 creatures (which each deal more than 1 damage) to survive.

Relevant rule:
615.7. Some prevention effects generated by the resolution of a spell or ability refer to a specific amount of damage–for example, “Prevent the next 3 damage that would be dealt to any target this turn.” These work like shields. Each 1 damage that would be dealt to the “shielded” permanent or player is prevented. Preventing 1 damage reduces the remaining shield by 1. If damage would be dealt to the shielded permanent or player by two or more applicable sources at the same time, the player or the controller of the permanent chooses which damage the shield prevents. Once the shield has been reduced to 0, any remaining damage is dealt normally. Such effects count only the amount of damage; the number of events or sources dealing it doesn't matter.

Hi,

Thanks for your question. Yes, they can survive by splitting up the damage prevention to prevent 2 damage from the 3/1 and 1 damage from the 2/1 with infect. You don't choose a source and then prevent damage from that source until you run out of damage from the source, then move on to the next source. You get to choose, for each point of damage prevention, which source to prevent the damage from. They'll be able to prevent enough damage from both creatures here to survive the turn.

Thanks,
Nathan Long
Magic Rules NetRep