Manual Description:

With superior senses of sight, hearing and smell, Jackals serve as scouts and assassings for the Covenant forces. They use plasma pistols and carry a strong energy shield to compensate for their physical weakness: a well-positioned Jackal can hold his own against several Marines, though grenades are effective against them. They stand approximately 5' 8" tall.

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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.

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