Slap on the standard seven COM passives: HP +30, both Resists, Strength +35, Role Resonance, Attack: ATB Charge II, and Siphon Boost II. The other three are up to you. Reaver is pretty sturdy by default so he doesn't need much in the way of extra defenses.
He's not in the guide because he's not top tier material, nor is he close enough to be noteworthy. The Guide does offer advice for customizing any monster you choose, and COMs are one of the types with a finished list of recommended abilities.
Well, IMO Reaver should replace Proto in the MIG. Reaver is categorically better. Proto is simply easier to build (and looks cooler). At the very least, Proto's entry should be updated to have him stop at Level 33.
I think the reason I personally never built a Reaver was to leave some passives open for infusion - in this case Critical: Bravery. Looking on it now though, that's a pretty poor reason in itself. Cactuar and Red Chocobo are the others with the passive, and you can have multiple Reds so... I honestly can't see what I would have been thinking. Why was Reaver never up for consideration?
I am actually confused now. Reaver is not in the same place as Tonberry, who is not even considered due to being a key infusion source. I don't think Reaver was ever even mentioned in discussions.
I think Reaver has just been flat-out overlooked. That in and of itself isn't unique, but what makes it curious is that other behemoths (namely Proto and Narasimha) have gotten plenty of attention.
Another reason is probably the fact that Reaver doesn't have Armor Breaker, but Proto's Armor Breaker results in a slightly weaker modifier than Reaver's Bloodthirsty, hence Reaver's categorical superiority. Narasimha's Armor Breaker is technically even stronger, but this advantage disappears during stagger, making Narasimha typically the least effective of the three standing behemoths, incredulously.
Proto probably also garners more attention since it is by default skewed toward a special type of build (Critical build) while Reaver is more or less a blank slate. Other than that, Reaver doesn't stand out - not conventionally, anyway. For one thing, I still think Apkallu makes for a better Area Sweeper despite not having 6 ATB.
All in all, Reaver deserves a "noteworthy" spot as a very good second-tier option.