Proof

 is the fourth chapter of Attack on Titan: No Regrets, written by Gun Snark and illustrated by Hikaru Suruga.

Overview
After the training, Levi, Farlan Church and Isabel Magnolia go to their first mission. Farlan is very scared of going out, but Isabel seems very happy and excited for going out. Soon after going out, Erwin orders the start of the Long-range Scouting Formation. However when reaching the Forest of Giant Trees, a regular Titan and a Deviant Type Titan attack the soldiers. Levi, along with Farlan and Isabel, manages to kill the aberrant, making Erwin see that he made the right decision.

Summary
Various months of training later, Levi Ackerman, Farlan Church, and Isabel Magnolia get their first glimpse of the world outside the Walls. They take part in a expedition launched the Shiganshina District to explore the world beyond the walls. Farlan seems scared and he cannot believe that this is happening. As soon as they go out, they are amazed at the beauty and freedom. Levi says Farlan that he will take care of the Titans to calm him, but he seems unconfident. Isabel is making fun of him when another member of the Survey Corps, Sayram, comes close to them, saying that killing Titans is not child games and that they are just criminals that cannot be compared to the soldiers that have died in combat. Isabel answers him that if she can kill a Titan, she would be better that all the soldiers that have died before her. The fight is stopped by Levi and the Squad leader Flagon, who says that it is necessary to keep the formation and forget the fight.

Not long after their departure a 15 meter class Titan is sighted.Part of the Survey Corps jumps into action leaded by Keith Shadis, working in unison to try and take out the Titan. At the same time a 20 meter class Deviant Type Titan appears from the forest. Another group of soldiers try to fight it, but the Titan manages to evade the incoming attacks and even kills a member of the group. With the formation beginning to fail, it's up to Levi's squad to get rid of the new threat. Levi orders Farlan and Isabel to cut his knees to stop him while he climbs to the Deviant Type Titan's back. With the Titan crippled, Levi jumps above the Titan and finishes it off with a powerful slash to the nape while falling. Levi's kill makes the rest of the Squad amazed, included Sayram and Flagon. The three friends congratulate themselves after disposing the Titan. Meanwhile Erwin Smith marvels at their accomplishment, satisfied that he made the right decision.