DelzGB

Within the virtual world of “Avalon Online”, Finn, or otherwise known as “Echo” within the game, was one of its greatest players. After logging off from a long session, however, he notices something awry outside of his apartment. 
The streets were flooded with chaos, screams echo through the night, and hell is loot loose. Monsters from Avalon Online are running through the streets, wrecking havoc. In the midst of this hellish awakening, Finn is greeted with the one gift bestowed to humanity in this dark hour:
[...Assessing life experience…] 
[Assessment Complete] 
[Name: Finn Thorne] 
[Age: 20] 
[Designated Class: Assassin] 
[Starting Level: 5] 
[A [Legacy System] has been bestowed to you for your accomplishments within Avalon.]
[Assimilation System Awakened.] 
In this twisted turn, as the world as it was once known is gone with the arrival of magic and monsters, the remnants of humanity must level up if they wish to survive. 
Finn is left in this dark, hopeless apocalypse, using his own experience from Avalon Online to best survive what seems to be an impossible task.

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Level-Up Apocalypse: Surviving With The Assimilation System

He nearly fell as another pebble struck his thigh, watching the crusher stop as he arrived within a dozen meters of the Arbiter of Stone.

It wasn't because the torrent of rocks inhibited him; no, Seung smiled brightly as he raised his fist before slamming it down into the ground–BOOM.

"Enough sitting there and looking down on us!" Seung ordered with a fearsome smirk.

A ripple of golden force tore through the quarry, propelling right towards the faceless humanoid of pink flesh. It produced a flash of kinetic power that erupted right beneath the designated [BOSS], knocking it off balance as the orbiting stones around itself were thrown off their key.

The disruption of the Arbiter brought the hailstorm to a halt, taking the burden away from the assassin who had struggled to move further.

"Hurry up already! Don't waste this moment–!" Seung shouted from ahead.

The Man Who Fights Alone