Shoot Fleshmorph in the orange area under his head.
Fleshmorph and Dreadnaught are the only ones that require specific actions to defeat them. Boss related achievements : Bosses will be found organically during your playthrough. As it would be hard to explain due to the nature of the level design, make sure to destroy all barrels in that level. The hardest to find is Nubia in the level 'Nuclear Power Station', in order to make her appear you need to destroy all the barrels in a specific room. Here is the light version of my achievements guide : Bikers related achievements : Bikers appear in levels, defeat them to unlock them.
