I recently watched Transformers: Dark of the Moon, in which humans attempt to get rid of the Autobots. After we send them off on the rocket the Decepticons attack, and the humans begin to fight back. It is my opinion that, although Autobots do have amazing offensive weapons it appears (in the movie) that the humans are able to hold their own against them, granted our loses would be incredible since you have to assume it will take at least a squad reinforced to take down one of them.
I haven't seen the most recent Transformers, so I don't know if that might be the premise. However, in the world of Transformers have the humans fought the Decepticons on their own? Do the movies accurately portray the ability to take them down with small arms?