First of all, shoutout to Lowenbrau whose original Russian version (Достижение "Средний брат" в ОДИНОЧНОМ режиме) helped me get this achievement (thanks to Google Translate). I asked him if he was okay with posting a translation since I have not found an English guide detailing the necessary steps for this particular achievement.

If it has not been made clear enough - this guide is giving you the step by step walkthrough to get the achievement in solo mode. It's way easier in online co-op, but if you want to do it on your own, here you go. I will go through each chapter where important choices are being made. As this got a bit wordier than I thought, I put dialog choices in list format to be easily spotted.

IMPORTANT NOTE:
You have to do this in the Theatrical Cut, otherwise you get to play different characters etc.

When Brad arrives at the Duke of Milan, the first dialog choices are to be made as follows:

• I had to work (RESENTFUL)
  
• (SAY NOTHING)
  
• Must be tough (EMPATHETIC)
  
• Go with your gut (SENTIMENTAL)

After the introductions, Conrad offers to drink beer with Brad. DO NOT accept:

• I get sea sick (APPREHENSIVE)
  
• I'll throw up! (SERIOUS)

As Alex, talk to Brad before diving:

• Wasn't sure we would (IMPRESSED)
  
• Get you something cool (ENTHUSIASTIC)
  
• Go through it all together (COMPASSIONATE)

Two things of importance here.
First, when Junior brings Conrad back down and says, "Be on your best behavior, little man!", choose

• Get off me! (RESENTFUL)

When Alex asks about his brother, Conrad will then tell him that Brad is hiding.

Secondly, make sure Conrad either successfully escapes on the speedboat or dies trying to do so. That way you will control Alex after the group escaped from the first room on the freighter.

Only one important choice here. When Julia, Alex and Fliss sit on deck and talk about Brad, have Fliss say:

• He might drown (ANXIOUS)

Once the group arrives in the big cargo hold, Olson will come into the room and set down the distributor cap. Fliss will go down to get it and Brad follows her. The relevant choices with Alex are:

• We need to be careful (DUBIOUS)
  
• (WARN BRAD)

Not really necessary, but to make things easier for you, DO NOT pick up the wrench with Brad. He can't hurt Fliss that way when they meet shortly after and he is still hallucinating.

When exploring the strange ballroom, make sure you open the curtain (blast door) with the lever in the small room. Once open, walk into the other cargo hold, past the coffin (crates containing the chemical) and open the door on the side to get some fresh air. Fliss will then see clearly again and be able to recognize Brad once he falls down from above (thus not endangering his life).

One thing that happened to me once was that I went outside too early, apparently. There is a background timer counting down while you are in this area and seemingly a second one starts when Fliss goes outside. If the second one runs out before the first one, she starts hallucinating again. Have her look at a few things before you go outside, just to make sure you don't mess up.

After the radio dies, the group decides who goes down the hole to turn the power back on. Alex declares that he will go, but Brad suggests it should rather be him. Alex acknowledges Brad's strength throughout this weird adventure, so have him let Brad go by choosing:

• If you're sure you're up for it... (ENCOURAGING)

Once Brad and Julia get up from the floor they crashed on, Alex asks if they are okay. Have Brad answer positively:

• We're okay (HOPEFUL)

You should probably get the achievement before the very last choice to make, but I wanted to make sure I got all options in here. I personally got the achievement at the best possible spot - when Alex "promotes" Brad to his medium bro - hence the title of the achievement.

Please let me know if you spot any errors! Also, if this guide helped you, consider leaving a positive rating.