Guardians have a superior arsenal with rockets that track, guns that poison, guns that deal splash damage so you don't have to directly hit them, guns that can mark his location, shotguns with superior range, etc. the grenades outclass halo's as well, we have zoning grenades, sticky grenades, and great AoE grenades. We have supers that can be used as the ace in the hole in close situations. We have a variety of melee abilities that can save us in close quarters, as well chief has a weak spot on his back that is a OHK without a perk equipped. Lore wise the ghost can gather massive amounts of Intel, is actually mobile unlike cortana, can revive the guardians, pretty sure are immune to bullets, can only be killed by darkness, and basically outclass cortana in every way. I love master chief but he would lose against a equal skill guardian. He's super weak to plasma: hard light, weak to spikers as well: thorn. There's a counter to everything he has and then some.