Skar Boyz are significantly more dangerous than regular Slugga Boyz, and on Hard difficulty a couple of these can take you to the cleaners in close combat. They need more hits to get stunned and endure a good bit of ranged fire. When they take sufficient damage, they go berzerk and scream Waaagh!, increasing their melee damage. You can recognize this by the red light emitted by their eyes and weapons.If a melee mob includes Skar Boyz, keep your distance until the mob is thinned down. Kite them using your pistol or main weapon, possibly even a grenade. Only fight a single Skar Boy in straight melee. If you don't need health, consider using a stun attack such as the Chainsword stomp and then shooting the enemies while they get up. See: Warhammer 40K Space Marine