The Beta Swamp Spider!
When they first entered the Spider Swamp, everything seemed rosy as together, they were able to keep all the threats at bay but their first encounter with a swarm of Swamp Spiders ended up being a nightmare.
It all happened in just a few minutes and they were still shaken.
Most of them, even Rex, this was their first time witnessing a close companion dying in action. Rex's original teammate was the 3rd soldier that died.
Though he could not refer to him as a friend, he already saw Gundogan as a fellow brother in arms so he was affected by his death.
They may have already undergone months of experience and training in the army, but this was still a lot to take in.
Despite the difficulties of accepting the 3 deaths, none of them stopped to mourn their deaths, they were soldiers and they were obligated to fulfill orders.
Discipline as emphasized by Sergeant Stones was one of the core traits of a good soldier, Rex and every other soldier here strived to develop this trait daily.