Mijissou no Last Boss-tachi ga Nakama ni Narimashita.

Young gamer Shuutarou is among the lucky few chosen to playtest Eternity, the latest VRMMORPG, and he wastes no time in activating his unique dungeon creation skill...at the exact moment when the game glitches out! Now he and his fellow playtesters are trapped, and their very lives are at stake. Shuutarou happens to end up in the one area that has yet to be patched into the game: the final-boss lair where the six Evil Overlords lurk. They promptly declare him their master—and with Eternity's strongest baddies on his team, Shuutarou is up for any adventures this new life will throw at him!

(Source: Yen Press)