You're saying that there are NO conflicts, NO deaths in practice, nothing. So, sorry, but that cannot be a "warrior" caste. Warriors DO fight, they need to practice, and when they do, accidents happen. Look what happens in most armies: They have fatal accidents while training.
Plus, even if they are exceptionally good at aiming at robots, or at "playing at war" (with virtual simulations, or make-believe weapons acting like true weapons, etc), which is acting at a "tactical" scale, they'll need to be good at strategy, logistics, etc, too.
I'm afraid that the best you'd get would be "armchair generals" with no real experience of warfare. Sure, you can study the campaigns of Julius Caesar, of Napoleon Bonaparte, of Edwin Rommel, or whomever you think was a great tactician and/or strategist, you can study Clausewitz or Sun Tzu (or the local equivalents), but you won't instantly be "better than all opposition".
Of course, there are workarounds to this. Let's say this "warrior caste" (which would really be a caste "interested in perpetuating the martial knowledge" - either because they think it is "cool" (a bit like gun-freaks, etc.), or because they think it might become necessary again someday) is able to train...
Cool. Would they have access to areas they are allowed to utterly ravage while playing war ? (have you seen the damage artillery creates ? Remember how you stated they had weapons of mass destruction which nearly annihilated their race ? Would the other castes accept their "playing" with such toys, even with the best of intents in mind ?)
If they do, how do they cope with the damage ? Are there weapon failures, needing dismantlement (minesweepers and the like), lest they cause fatal accidents later ? Do they replant trees, re-create the ground like it was before, etc ? How do they avoid casualties while playing with real ordnance ?
If they don't, how do they train properly ? Do they have access to ultra-realistic simulations, allowing for full consequences of each and every action, just like it happened in the real world ? Do these simulation work on EVERY sensory level ? Visual and audio, as we know in our 3D shooters, even in VR, are utterly insufficient if you want to train soldiers, more so if you wish to train ELITE soldiers.
I mean, they do not train the stamina, do not prepare you for the smell, the tactile sensations (ground rumbling because an explosion shook the ground near you, even if you're behind cover, you might lose your footing and be thrown to the ground, air pressure will change too, etc). Would the soldiers be prepared to see REAL corpses from deceased teammates ? The training MUST be mental AND physical, work the reflexes and the instincts, build fast reactions, etc...
So, unless they really have such technology, and currently compete against themselves to stay in top shape, thus having to constantly revise their tactics, I highly doubt they'd be the elite force you need them to be.
Also, don't forget that they would fight amongst themselves (even if only "for fun", not for real), and that a lot of the strategies they would have devised would probably only apply against an adversary they know.
Tactics evolve because threats evolve (along with technology, and new offensive tactics). If your opponent uses infantry, you devise means to destroy said infantry. If it uses vehicles, you develop anti-vehicle weaponry. If you think they will come with aircrafts, you create anti-aircraft defenses. That's because, as a human living in the 21st century, you KNOW those are real threats.
Now, would you build, as a human in the 21st century, a defense against "giant space worms" ? Or against a Death Star taking position near the Earth, ready to take it out ? You wouldn't.
Of course, you lack the technology to do so, but even if you knew how to build something very powerful, able to blast the Death Star to pieces, if it were to come near the Earth... would you do so ? Considering the material cost of such a weapon, and the fact that most sane people would tell you that the Death Star is only a fictional machine, and, thus, has no real chance to come and attack us, I'd wager that nobody (or not enough people, at the very least) would be willing to build that super weapon.
My point, here, is that even if you train elite people to the best of what their race ever knew, they probably wouldn't be adapted against a threat they didn't even envision. If aliens land and field an army of a type we "understand" (with grunts, vehicles, flying things, weapons that shoot things in a straight or ballistic trajectory, etc), we should be able to field a defense quickly, and it might even be somewhat effective - depending on the offensive and defensive capabilities of the alien's gear.
But if those aliens are able to teleport at will, for instance, then everything we know about strategy (establishing defensive perimeters, scouting for positions, creating chokepoints, etc) becomes instantly moot.
If their weapons kill you without an understandable mean, how will you try and defend against them ?
So, to sum it all, I'd say that you may maintain a force which has the POTENTIAL to become an elite force. But, for it to REALLY be that elite force, it will need to learn about its foe, and that usually will entail casualties. Think of a civil-war era general, encountering a modern armor in the field, for instance. Maybe they would be able to vainquish it in time, but you sure could expect a lot of casualties in the meantime.
And if you reverse the trope, even modern troopers could die fighting a stone-age tribe. Maybe not because of the warriors themselves, but, say, because they're not immune to a disease, or to the toxins of a local plant or animal, or whatever...
Also, if the opponent is VERY underestimated (say, you're mowing down thousands of enemies per hour, without losing a man), your soldiers could become careless, see themselves as "invincible", and start goofing around (starting wagers, skill competitions, kill counts, etc... leading to possible risk taking). Imagine a guy taking a "dare" selfie, back to the enemy, or other stupid behavior.
I'll stop here, as this answer is probably already too long, but I hope my points make sense.