when I started in TKD, there were two brothers who were blackbelts; both were big, strong and skilled; one was technically amazing; the other was just crazy; when they sparred you, they would knock you around at a level just beyond where you were at, but they did it concertedly, they hit hard, and they just kept coming; facing these guys, you came to know the meaning of fear, because they would hurt you - not too bad (unless you showed some spirit, then they really ramped it up, lol), but just enough for you to be on your toes, and have to really swallow that fear to get out on the floor w them; turns out that certain people wouldn't spar them (I know this because the head instructor told me, years later that every so often, someone would come to him and request to not be paired w either of them); in my mind, having to face these two was an exercise in martial spirit - I hated it, but in my mind there really wasn't any option other than facing them; so you put on your best game face, and died with honor...

that's one way;

the other, less combat oriented, is to have people do overnight wilderness survival outings - give them minimal items, they have to stay out for a set period of time, etc.; from personal experience, being alone in the woods, even though you know they know where you are, etc., is challenging; this builds martial spirit, albeit in a different way