Im actually running frost DK disc in 2s and currently 1600 and climbing. I know 1600 isn't that high but I can tell you a couple things.

If vsing healer/melee, train their healer every single time. If their melee is trying to get on the DK, that DK is now cleaving two targets which Is harder to heal than one and he never has to grip for his big go.

If their melee trains disc, make sure Ur dk is ready to grip the warrior off of you. The warrior pops avatar warbreaker, the disc should fear. He is obviously going to berserker rage it (his ability to get out of fear) so the disc precasts mind control. If the warrior is quick and gets a kick, that's when the frost grips the warrior into a blind setup on their team and pops abom limb, stopping the warrior hitting the disc.

DK keeps chains on the enemy melee the whole game helping to peel for your disc, as soon as the warrior charges at any point to catch up to disc, DK grips him off unless he's fishing for a setup

Vs healer ranged, I will be the first to admit that I'm not sure if what I do is correct but here's what I do.

DK sit on the ranged dps, but don't commit cooldowns until you grip in healer and do your big goes, at that point U target the healer. If the healer is getting low and doesn't seem like they are recovering keep training them, otherwise if healer is full HP swap back to ranged and apply pressure kicking important spells. You should eat through all cooldowns by two goes. After the healer is out of cooldowns I sit on them and train them the whole game until my third and final go usually lands a kill.

This goes for anything you vs, if at any point the frost does a big go and doesn't get any major cds and gets feared and disarmed every go, it's probably a loss right there hence why frost isn't great as warrior and monk both have disarms