Relentless Medal and Killstreak Rewards in Black Ops 2

Completing all of the levels will make the later parts of the campaign easier, because

China will not go to war against US and will send military help.

An extra strike team mission will let you fix you mistake you can make during one of the campaign mission, specifically

Failing to save the scientist lady from abduction.

Here is a video that talks a little bit about it. It talks a little bit about setup and what map to use. It recommends sticks and stones. Other objective based maps will do as well like things like dominations.

For setup he does:

• reflex scope
• lightweight (you can pick something else ghost is also good)
• scavenger (wont work if you kill with explosives)
• dexterity (crossbow aim is slow so its important)

Hopefully this is enough information to get it yourself. If not I hope it at least gives you enough motivation from this to get it.