If we add in flood, precursors, or fore runners then each solo the whole destiny universe. Elites have training slightly below that of a spartan but I'm not sure on there current numbers but depending on the numbers they could also solo. The hardest would be vex due to time travel and teleportation but all halo enemies can warp instantly. Time travel for vex only works in the vault according to grimoire. So I think halo pretty easily wins.
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Well time travel for atheon was locked to the vault, there are still other vex capable of time travel outside of the vault (hence skolas pulling the house of wolves through time before you catch him)
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That was him reversing engineering the technology. The vex themselves don't actually time travel out side of the VoG because they want to do it in controlled locations for safe reasons.
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Though in the grimoires it reveals the abilities like containment fields, oracles etc are forms of time travel to "erase" what they feel shouldn't exist and Qodran is capable of using this in the prison of elders. The vex networks are also in a sense time travel tech since as ghost put it they instantly travel anywhere through they're networks. Either way makes for very dangerous tech