Instant Viking - Assassin's Creed: Rogue - Remastered

Hit 5 Enemies with a Berserk Grenade at the same time - 20

I recommend herding enemies in a group by using the Firecracker Dart with the Air Rifle so you can group at least 5 enemies together. Once there is a group of at least 5 Enemies, use the Grenade Launcher to fire a Berserk Grenade, causing the enemies to go berserk.

Viking Berserkers were noble warriors who could work themselves up into a Berserker's Rage. While in this angry Berserk Rage, the warrior would be able to just concentrate on causing damage, bypassing the feeling of pain.

Air Rifle with Grenade Launcher Attachment