I think that the only real advantages that I could give to Spartans in that situation would be flat-out movement speed. That, and if each of them had a Smart AI plugged into their systems a la Cortana and Master Chief, as that sort of data-processing and added situational awareness could be very helpful. Basically to take down an Astartes, you'd need at minimum three Spartan-IIs: One on overwatch with their sniper rifle/relaying information to the other two (which, admittedly, is fairly good for non-reactive rounds; if it can punch straight through an Elite's shields and armor and keep going, I think it stands a reasonable chance of penetrating Power Armor), and two to tag-team the Astartes with plasma swords.
Even then, I'd only give them maybe a 33% chance of success, if they had the element of surprise and got the first strike. All it would take for the Astartes to kill any of them would be one or two hits with a bolter, or a single solid hit with any heavy or melee weapon, by my estimation. So if you had the greenest Astartes you could get, with nothing but a chainsword and bolt pistol, matched against a veteran team of Spartan-IIs used to working together, well armed, supported by a trio of Smart AIs, with surprise and pre-established ambush positions... maybe.