Your Party Crasher will kill both of your opponent's Hill Giants, and die. You will gain 6 life
From Mark Rosewater's Unstable FAQAWASLFAQPAFTIDAWABIAJTBT! (That's "Frequently Asked Questions As Well As Some Less Frequently Asked Questions Plus a Few Things I Doubt Anyone Would Ask But I Answered Just To Be Thorough.") (Emphasis mine):
During each combat, there's a time when attackers are declared. Normally, only the active player declares attackers, but Party Crasher doesn't play by those rules. During the declare attackers step of your opponent's combat phase, after they're done declaring their attackers, you may choose to declare Party Crasher as an attacker. After you declare blockers, the defending player can declare blockers for Party Crasher. All combat damage is dealt simultaneously, unless a strike ability (first, double, last, and/or triple) is involved.
During each combat, there's a time when attackers are declared. Normally, only the active player declares attackers, but Party Crasher doesn't play by those rules. During the declare attackers step of your opponent's combat phase, after they're done declaring their attackers, you may choose to declare Party Crasher as an attacker. After you declare blockers, the defending player can declare blockers for Party Crasher. All combat damage is dealt simultaneously, unless a strike ability (first, double, last, and/or triple) is involved.
