Spartans are bred for war and have much better training and physical attributes than guardians. Spartans have gear that counters or matches most of the guardians supers and abilities. They have armor lock to counter literally every super a guardian could use. A 6 man spartan squad (especially one up to noble team's caliber) would beat a 6 man guardian fireteam due to their superior training alone granted it'd be a close fight. Plus if the guardians got master chief then it'd be no contest.