Jackals may well take the cake as the most annoying enemies. As the manual says, they're not particularly strong; but their energy shields are, for all intents and purposes, impervious. So unless you're very good (and can, for instance, slot a shot to hit the Jackal through the small gunport on the shield), they can be very difficult to get rid of. Grenades are indeed effective, as are melee attacks (which will disrupt their shields.) Also, their shields can be overloaded by a charged plasma pistol shot.