There was a time I'm sure people found a way to solo it, just unsure if that was a glitch, using the exit corridor of the room the boss is in or something
You can solo Direfrost Keep. There are several threads on the subject here.
I just ran it solo last weekend.
In short, clear everything up to the plates, backtrack to the room just east that shows the small tunnel that goes to the room under the plate room.
There will be a locked gate. Walk up to the gate and you can "use" the lever on the other side to open it.
Make your way through the tunnel to the room under the plate room.
There is also a locked gate there which you can not open but you can aggro the boss from near the gate.
Kill the boss through the gate.
Backtrack to the start of the dungeon and use the portal to teleport into the room, from there you can proceed like normal and finish the dungeon.
PS: If you are running on vet and you have a pet out, put the pet away for the final boss and the guy that is the bosse's assistant (apprentice?).
They both have an attack that siphons health from you and you can interrupt it, but if you have a pet out, they will siphon from the pet instead and you can not interrupt that. On normal, you can dps them down anyways, but on vet they can heal up to 100% easy that way, so it's best to put your pets away for those two fights.