MTG Salvation

Spells and abilities on the stack resolve last-in, first-out. When a player casts a spell or puts an ability on the stack, other players can "respond" to that object on the stack by casting instants, spells with flash or activating abilities. The spell or ability put on the stack in response to the first spell or ability resolves before it. That is how, for example, a card like Cancel works: you cast a spell, putting it on the stack; in response, I cast Cancel targeting your spell, Cancel resolves before your spell, countering and sending it to your graveyard before it can resolve. In your example, Dragon Blood's ability resolves before your spell that deals damage, so the Tusker has 6 toughness at the time it gets dealt 5 damage and thus it survives.