shock troop
/ˈʃɒk ˌtruːp/noun
1Troops specially trained and equipped for carrying out sudden assaults, especially against enemy strongholds.
2 Figurative and in extended use, especially the driving force of an activity or movement; a group leading a vigorous challenge or defence.
3In singular A detachment of such troops.
Origin
Late 19th century; earliest use found in The Times of India. From shock + troop, after French troupe de choc and subsequently also German Stosstrupp.