Yakuza: Dead Souls

After completing all 60 Substories (See Substory Guide and Checklist) and Subterranea Missions (See "Kamurocho Spelunker" trophy) for each character, switch and play as Kiryu and you will receive a message from Kamiyama, requesting you to meet him in his truck. A short conversation will play out and then a message from Amon will arrive, requesting you to meet him in the Underground Tunnels.

If you are following the Road Map, it's best to do this in Premium Adventure after completing your Dead Souls and Extra Hard playthroughs where you would be sufficiently levelled by the time you get Amon's message.

First, upgrade your Anti Material Rifle as much as you can, the strongest being the Dragon Rifle (Schematics obtained by completing Kiryu's Substories). Equip yourself with your strongest Armor available and any defense boosting accessories. Bring along 4-8 Toughness Infinity and 100 Anti Material Rifle ammo.

You may bring a partner along when you enter the Quarantine Zone, I brought along my highest level partner and equipped her with health boosting abilities. Head to the manhole in Taihei Blvd East that Kiryu used for his Subterranea Mission, here you will receive a third option called "Go to Amon". Do so and it transports you to the lowest level immediately.

A cutscene ensues, and you are attacked by Rasetsu Amon instead. This zombie has 3 attacks:
1. Rasetsu fires exploding balls of light
2. Rasetsu catches you in a grip and drains your health
3. Rasetsu fires a red-black wave that deals A LOT of damage and may instant-kill you if your health is 50% or less.

There isn't much you can do with #1 other than dodging in quick succession (Rapid Evasion ability helps). You can reduce the damage received if you caught by #2 by quickly tapping the button. For #3, you MUST always stay away from Rasetsu. Maintain a long distance between the both of you. Every time you see Rasetsu glow red, he is about to use this attack. Luckily it has a very narrow range and can be sidestepped if you are quick enough.