Bokurano, Hoshi no Samidare Recommendations
An alien guide appears and tells a small group of humans that the fate of the world lies on them. A series of duels between them and foreign entities will take place, and loss will result in the destruction of the world. But this is a group of flawed humans, and they are neither united in their cause nor equal in their abilities. How they deal with the continuing battles while losing allies and finding out more about the nature of the fights is the focus of the story.
Samidare is more interesting to watch, as everyone participates in the battles, but they
Both feature a group of people that end up being forced into a must-fight scenario in which, as a team, they must defeat the enemies that are beyond their comprehension one at a time with breaks before battles. Their loss results in the destruction of our planet. Both contain heavy character development while dealing with the psychological state of the participants.