Badass Army: The Imperial Legions try very hard to play this one straight when they have to.  Because of the low-growth, low-manpower demographics of a long-lived species, they generally prefer to eschew most army-type fights in favor of the Sneaky Bastard special-ops method of warfare, or better yet, the the-leaders-of-our-enemies-all-dropped-dead-in-totally-unrelated-accidents-that-cannot-be-connected-to-us-three-years-before-the-war-would-have-happened method of not-warfare.  But sometimes you have to have a real war anyway…

…which is where they figure they need to make the most of the limited number of sophont resources available, by virtue of equipping them all with genetic and cybernetic enhancements (see: Super Soldier), training them rigorously in their own version of The Spartan Way, including in the incredible coordination and discipline that comes from fighting as a conflux (temporary group-mind), and then equipping them lavishly, which means power armor even for regular infantry and combat exoskeletons amounting to walking tanks for the heavy kind, a plethora of Mecha Mooks and Attack Drones attached to each individual soldier, and liberal use of heavy weapons up to and including nuclear/antimatter hand grenades.

After all, when you don’t have numbers, you might as well plow that budget into extended training and capital equipment, right?  The Legions themselves may be small in number, but they work hard to make facing them like facing an army made up entirely of elite troops, with a weight of metal on the field a couple of dozen times higher than should be reasonable for that number of people.