The damage the Giant Slayer skill deals to the Clan Boss is most likely capped far below the 2% that the tool tip states it deals to the "Boss' MAX HP". If 77636 damage would be 2% of the Clan Boss' overall HP, it could be taken down by a single player as it would only amount to 3,88 Million. Bad-el Kazar's buff probably works as intended, it's just that the Clan Boss' special rules for hard capped Giant Slayer, HP Burn and Poison dmage override any other damage modifications.