I'm a die hard Halo fan, but I'd have to go with the Guardians for this one.
The simple fact that they can be revived via their ghosts makes them a force to be reckoned with.
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But if the ghost is killed, which is easier than one might think, a guardian can't respawn. Also, Spartans wouldn't die to a guardian *ever*, so the fight would just be obliterating the guardian hard enough to destroy its ghost, with little resistance from the guardian.
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If a hive god cant kill the ghost, who's to say a Spartan can?
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Petra Venj killed several ghosts with a basic airstrike, a spartan II could kill a ghost easily.
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Not my ghost mf.
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Kay...?
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But, if the ghost is killed, the guardian remains dead.
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If the ghost can survive all of the bosses from we've faced thus far, including an undead space zombie god, I think the ghost could survive a group of Spartans.