Soul Hackers 2

This achievement ties with the main story progression of the game, unlocked when you defeat Kaburagi in the room located west to the entrance of Northwest Path Portal in the Middle Area portion of the Central Line subway tunnel complex.

For preparation, it's a good idea to do some grinding to level up both your summoners as well as your demons for a variety of reasons, from bolstering the stats for the former to unlocking key attacks for the latter, particularly moves targeting weaknesses for Kaburagi and the pair of demons he will bring out (Kurama Tengu and Leanan Sidhe). You'll want to prioritize doing your grinding in the Upper Area of the facility, where demons are leveled higher, and you can gain more XP and level up quicker, especially for your demons. (I recommend leveling up your summoners to between level 18-20, and demons to at least level 15.) It's also now a good time to experiment with fusions in the Cirque de Goumaden located in the Roppo Realm to create stronger demons with the moves you need and get you closer to the ideal lineup for your party that can do the most damage and hold the greatest endurance against Kabruagi and company. Don't forget to upgrade your COMPs in the name of boosting your base attack stat and make use of augments with Fire, Electric, and Ice elements, which are the weaknesses for Leanan Sidhe, Kurama Tengu, and Kaburagi respectively. So accordingly, make all the equipped demons in your party have attack moves with at least one of these elements in their movesets. And lastly, eat a meal of Baked Sweet Potato in the Safehouse before leaving to face the boss, it'll give you a temporary boost in the INT stat (and more importantly, the higher the base INT, the higher the boost goes!), which amplify the power of magic attacks, and that'll really help you at the start of the battle. You can buy them at the Delamancha in Karakucho for 500 yen each. For those concerned for what Kaburagi and the demons bring for attack types, Kaburagi brings Gun and Electric attacks, as well as an Almighty all-party attack called Execution that always follows after an Overdrive Mode from his previous turn. Kurama Tengu brings an all-party Force type attack as well as a medium Physical move targeting a single member. Leanan Sidhe holds a Ruin type move called Lullaby that targets all members with the chance to inflict Sleep status. But the real dirty moves in her skill set are Abyssal Veil which removes the Ice element weakness from Kaburagi and stays in place until she's defeated (she will deploy this move first after your initial turn), as well as the Media spell that heals all members of her team. So defeating her first should be first priority. And now, onto the battle.

Now at the start, before attacking, press for Commander's Skills and scroll down one slot and select Stack Optimization. This will double the count for each attack where you exploit enemy weaknesses toward the stack that will add to the power of your turn-ending Sabbath move. (Example being, each attack without Stack Optimization adds a count of 1 to the stack, but with it, you gain 2 to the stack for each such move.) If you do everything right, from a standpoint of leveling up your summoners and demons, equipping the right augments to your COMPs, consuming the Baked Sweet Potato to boost the INT, and then enabling Stock Optimization for your first turn and successfully attacking weaknesses for each turn of all four party members, you'll get your Stack count of 8, execute your Sabbath move, and take out close to 90%, if not the entire lifebar, of at least one of Kaburagi's demons while he loses around half of his lifebar. If both demons should survive, take out Leanan first, then Kurama Tengu before aiming to finish off Kaburagi. On his first move, Kaburagi will call for Accelerate, giving him the ability of multiple actions within a single turn. You should be able to work around this where you won't need to waste much in the way of any turns for healing or removing the Sleep status if you've leveled up to where I suggested. In the turn that he calls for the Overdrive Mode, if on average your party members have lost at least 25% of their health, have one party member use a healing move from an item that can heal all members if possible. If not just guard to mitigate damage from Execution (All my members were at level 22 when I took on this fight, and lost close to 40% of their health from taking the hit without guarding beforehand. You could still survive at lower levels if you're closer to full health, but guarding is always the smartest choice!). After it's done, he'll need a turn to recuperate. Use this opportunity to pound on him. Keep at this until he's defeated, and you'll get the post-battle cutscene where the achievement should pop!