The trigger goes off when attackers are declared, and it goes on the stack at the next possible moment, which is right after attackers have been declared before any player receives priority in the declare attackers step. The game cannot continue to the next step/phase until the stack is empty and all players have passed priority in succession. So the trigger has to resolve, you draw a card, and can even play that card (if it is an instant, has flash, or has an activated ability usuable from the hand) in the declare attackers step, before the defending player(s) can declare blockers. And no, the Packleader doesn't have to survive longer than the declare attackers turn based action being completed. It could die before the trigger even makes it to the stack, and you'd still get the card.
Also, does Pack Leader have to survive combat to draw a card?
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)