That Time I Got Reincarnated as a Slime (vol 29)

Using all his skills, Soei one of Rimuru's oldest and most faithful companions must fight the assassin who brought a sniper rifle to fantasy world! Then, when the smoke of battle clears, Rimuru and his friends join an expedition to the lost city of Amirita, a mysterious ruin full of treasure and danger. But in the darkness of the lost city, evil forces wait for the chance to strike. Weak- ened by the deadly dungeon, the slime finally comes face to face with Maribel, his mind-controlling enemy...and with Yuuki, Maribel's most powerful pawn!