I'd guess that SP has killed a few big-ass bastards in his time. It should be possible for him to do it again, especially with the super-acid-rust stuff that is probably actually nanotech. [Why didn't he use it more in DotM? He could've won the day!

] Overlord might have protection against the rust, such as element 113. He also has tremendous strength and firepower.
Having a jet mode, O can swoop down for a massive surprise attack. SP would survive the first strike but the second strike would be upon him soon. O has a secondary tank form that would shell SP from a distance. I don't know for how long SP could withstand this battering. He is tough so I guess he might attack Overlord's tank section head-on and try to rust it. At this point, O would rush to unite his jet and tank for maximum power.
If the rust works against O, he loses but he might be able to kill SP before he goes.
If the rust doesn't work, SP will have to do this the hard way. He can slash away with that big sword (which Thanos ripped off later) but O will flood the area with ordinance and eventually cripple SP. O would win but with severe damage, so he'd be vulnerable to other Autobots in the area or even Decepticons with scores to settle.