Like a Dragon: Ishin!

There are 8 Revelations, 2 for each combat style. To have a "Revelation" you must acquire a certain book, then when you look at the book's description you'll see what you'll need to do in combat to have a chance of having the revelation. When in combat(with at least level 1 heat) and doing what the book says, you have the chance of the Heat Action option popping-up on the left-side. Press /F to trigger. If you've had the revelation instead it going into the Heat Action you'll get a quick scene with Hajime/Ryoma in a thunderstorm where you have to do a 1-button QTE. If the QTE is done correctly you'll now go into the Heat Action. This heat action IS logged in the completion log after this Revelation, and for this achievement you are not required to place an Orb into this ability, you will need to for the "Limit Breaker" eventually. The achievement will unlock after you've had all 8 revelations. To find out whether you have a book or not go to your Inventory and go the "Valuables" tab with / or C/Z. If you have one there it'll be called "Book of Revelations: " followed by a title. Below are all 8, their titles, move names, method to acquire, and tips for triggering the revelation.

Brawler Style:

Book of Revelations: Strike

Ability: Essence of Grand Bruising
Acquisition: Reward for Advanced Room 10 in the Scarecrow Chateau
Trigger: Hold when an enemy attacks to trigger "War Cry Counter". After countering there is a chance for the heat action to show up.

Book of Revelations: Throw

Ability: Essence of Reverse Throw
Acquisition: Complete the final Brawler fight against Komaki.
Trigger: This one can be difficult because of Essence of Grand Bruising and a couple other abilities that can trigger heat actions when guarding or when you're about to be attacked. I found the best way to trigger this is to stand still when you parry a move by hitting /Q when an attack is about to land. If Hajime/Ryoma counters and is still facing the same direction, it won't activate, but after hitting /Q if Hajime/Ryoma has turned to their side then it worked, and you should hit /F to trigger the revelation if the heat action icon shows up.

Swordsman Style:

Book of Revelations: Smash

Ability: Essence of Mincemeat
Acquisition: Complete half(8) of the Wanted Men requests. The eighth mission should be "Touken Clan: Yukimitsu".
Trigger: Break opponents guard by attacking them until their guard shatters, eventually the heat action icon should appear.

Book of Revelations: Flash

Ability: Gleaming Scythe
Acquisition: Complete the final Swordsman test against Ginryu.
Trigger: You must have placed an Orb in the ability "Splendid Skill: Fair Weather". While guarding by holding down /Q, activate "Fair Weather" by hitting /F right as the attack would land. Successfully triggering "Fair Weather" gives a chance of the Revelation to occur.

Gunman Style:

Book of Revelations: Gust

Ability: Essence of Old Scratch
Acquisition: Buy from one of the material merchants in Unyuan(Gunman Dojo) for 5x Carved Wooden Bears.
Trigger: You must have an Orb placed in the Ability "Texas Two-Step". Perform a Texas Two-Step tackle by pressing /LMB after doing a forward evade(/Space) into an enemy. If you tackle the enemy you've done it correctly and the heat icon can show up. This one can be confusing as the book just says to do a Texas Two-Step but it won't trigger unless you to a Texas Two-Step tackle.

Book of Revelations: Spree

Ability: Oh-Be-Joyful
Acquisition: Complete the final mission with William.
Trigger: Perform "Jumping Shots" by hitting /LMB after running towards enemies. There are two parts to Jumping Shots, where you do a high-knee jump and where bullets shoot after. You need to be doing a Jumping Shot into an enemy so they get hit with your knee instead of the bullets. Once the knee connects there is a chance for the heat icon to show up.

Wild Dancer Style:

Book of Revelations: Evasion

Ability: Dance of Mourning
Acquisition: Reward for Hell Room 10 in the Scarecrow Chateau
Trigger: Evade near sword-wielding enemies using [cn_a]/Space and you should eventually have the heat icon show up.

Book of Revelations: Frenzy

Ability: The Bloody Wheel
Acquisition: Complete the final Wild Dancer test against Ginryu.
Trigger: Perform only two strikes of "Phoenix Frenzy" using /E on an enemy then look out for the heat action icon.