Does anyone have any experience with or knowledge of anyone who has filed an Article 138 UCMJ?

While providing legal assistance as a mobilized Army Reserve JAG, I assisted an O-6 in filing one against a GO. Client confidentiality (and common sense) prevent from detailing it here. Similarly, I am not going to provide legal advice. The COL ultimately prevailed, but it was a long ugly process. The convening authority became the CAR shorty thereafter. As for me, although I cannot prove it, I believe it's a primary reason reason I was passed over for MAJ. I was prior enlisted and so just opted to retire a few years later.

In addition to consulting the IG, I make an appointment with legal assistance. There may be other options. If possible, see if you can get a JAG from outside the chain.

So the complaint itself can be easily filed and

One has to understand the blind scales of justice, per the UCMJ. One pan of the scales is reserved for EM's and Jr.Officers......the other pan is for Officers 0-6 and above.
Let us review a couple of Air Force cases a few years back on the UCMJ that tried Gen. Fiscus, JAG- USAF, on Article 92, unprofessional relationships (10 years), conduct unbecoming and officer, fraternization and obstruction of justice.
Punishment-Article 15, reduced to colonel and received a reprimand, drawing full retirement pay.
SSgt. Jones, Unprofessional relationship, (two months), violating a no-contact order.
Punishment-Special Court Martial, reduced to Senior Airman, received a reprimand and fined $1,000.
What say the scales of Justice?

yerp

When it comes to anything having to do with going head to head with the COC, you better have a lot of time to invest in research. You can forget about your unit JAG office because they will tell you "I work for the best interest of the Commander" so they are useless. That's the other thing, the Commander has a JAG office all to themselves who can work on his defense ALL DAY LONG, you don't have that. All u have is Trial Defense Services and that's probably a CPT or LT that doesn't have a lot of experience. IG is a good approach but, after they investigate, the Command is going to know immediately why and who it's coming from. 15-6 investigations need some serious revisions as well. I say this because most of the time, the investigating officer is a new guy that still has to prove themselves to the Command. They are picked that way on purpose because the investigating officers recommendations could and will have an impact on the career path the Commander chooses for the individual AFTER the 15-6 is complete. I've personally witnessed a Brigade Commander O6, have a 15-6 packet returned 3 times to an investigating officers because he didn't have certain verbage in the investigation so the Commander could read an article 15 to the Soldier being investigated. Granted the investigating officers can go to IG as well BUT as mentioned before, his career will take a nose dive once the investigation is complete. The other problem with IG, is they want to have sworn statements...Good luck trying to find anyone with rank that will put their name on a piece of paper acknowledging that the Commander was wrong. They have their careers to worry about too. This course of action you are considering will work if you have all your ducks in a row, IN A TIGHT ROW but, you will probably be all alone. Don't look for too much support from people in your unit, IG and a lawyer will be your best bet.

I'd venture to say that most everyone of these responses are words of wisdom and I'd take many of them to heart. The 2 I'd keep closer though would be the one by COL John Hudson who has IG experience and those comments that question why a PV2 would seek to use such a powerful tool like AR-138. Unless this grievance is a multiple time offense and you are not the only soldier involved I'd recommend taking a different approach to resolve the issue. If all else fails, or has failed already, then and only then would I recommend going with AR-138.

The previous respondents are correct. Filing an IG complaint is the way to go. However, the IG is not a panacea. All parties are investigated as part of an IG complaint. The investigation can result in charges against the person filing the complaint. Finally, remember that the IG is in place to support the command. About 90% of IG complaints end in favor of the command. Do what you think is correct, but be mindful these factors.

I would SERIOUSLY ensure that you have addressed whatever the issue is, with ALL levels of the food chain. Regardless of whether they helped or not.
CSM below is on the money. Make 150% sure you have shit to back it up with, preferably documented in paper, EMs or individuals of good standing that will vouch on sworn statements if required. Otherwise, it may come across as quibble or a " he doesn't like me" type situation. Don't do that.
The IG should be your last resort.
I assume you have already addressed this with your CO 1SGT, PLT LDR, the CO and the BN SGM, right ?

I saw this like requesting mast?

Yep. Got held up on promo list for year.