Pretty much every GDL battle but especially Helm and the battle of Glengarry in "Day of Heroes". As others have stated, Wolf's Dragoons vs House Kurita in the 4th War, the essentially held the entire Davion border alone, front Davion troops to attack the CapCon. Specifically the final battle on Crossing, when barely a regiment of Dragoons, reinforced by a Davion unit, hold against 4+ regiments of House Kurita forces, forcing them off planet when Zeta Battalion arrived to save the Dragoons. As an article in "Battletechnology" put it, decimated is when you lose 1 in 10 of your soldiers, what do you call it when you lose nearly 9 in 10?