Aight. So for the purpose of this, I'll be assuming
a Urophio is the ceremorph in question, since that's the first Low 7-B one that comes to mind.
The main forms of attack are tendrils, which are used to try to grapple the target and excavate their brain cavity, and mind blast, which deals pretty absurd psychic damage and leaves a target stunned for the foreseeable future. It also has extremely potent mindhax but the actual usage of that mindhax is limited (should be considered roughly the population of a country in potency, via scaling to Strahd, but it can only be used to give suggestions on actions, not outright control).
Doomguy has vastly better LS (Class M) and should be able to tear the Urophion a new hole if they try to grapple. The issue, then, comes in with mind blasting him, for the most part. That is not an issue, though, when one considers the range advantage of Doomguy- at worst the guy is hundreds of meters away with standard weapons. Being fair, that range advantage is wasted on him, since he'll just immediately begin charging, but while he's charging he's still unloading an arsenal equivalent to a smaller country on the Urophion.
Based on all this, I do believe Doomguy could take him out by right of blasting him to kingdom come with range advantage, while all Urophion can do is use lower end mind fuckery to get him to do stuff. Suggestion ain't incap, just temporary fixes.