I don't really use the training grounds, but this is how I train them.
Just fight some taiga bandits (sea raiders when they get above the first tiers) and order your archers to hold position on a hill. Put your infantry in front of your archers a bit down the hill so that they don't block the sight and wait for the enemy.
You might have to repeat this a few times, but you'll notice that a 20+ group of bandits will be no big deal and you won't even lose anyone if you use your tactics properly. My Nord party slaughters any bandit group when we go training, they won't even get to clash swords if there are below 20 enemies. Just get a big enough party (at least 20 of the kind you want to train), my party is 20 archers and 20 infantry.
With cavarly, use the same tactic for archers and infantry, but put your cavarly on the side to flank the enemy. This might make some of the enemy come after the cavarly but then they are just easier to kill for your guys. When the enemy is getting near your infantry lines, make your cavarly to charge. This way you lose very few trainees and they all get some kills.
I hope this helps
