served as a member of the rearguard in the retreat
「撤退戦でしんがりを務めた」は
served as a member of the rearguard in the retreat
のように言えるかと思います。
rearguard は「しんがり・後陣・後軍」
retreat は「(軍事での)撤退・退去」
例:
He served as a member of the rearguard in the retreat.
「彼は撤退でしんがりの一員として務めた。」
He fought off the opposition at the rear of the troops as his unit evacuated from the war.
「彼の部隊が戦争で撤退する時、彼は軍のしんがりとして戦い敵を撃退した。」
ご参考まで!