High Level Beasteary

Going over the high level monsters and beasts for my team to go toe to toe with I'm finding that I have to adjust most of them a good deal to make them any sort of a challenge.

In my last campaign I had a great little hook where there was a dark necklace that attracked the undead to it, so our Rogue who just "couldn't help himself" put it on, which made it so that the rest of the team had to constantly protect him. That made it a really fun game since every time he would see something he would run and hide and all of the undead would charge after him... so funny.

Anyways, on the last fight they went up against a Death Knight (with the optional Long Sword of 9 Lives) and he went charging after the Rogue. Unfortunately, he wasn't able to even reach any of the team members since he won intiative, but he was able to do a bit of damage with his AoE spell.

First round for the players and the Wizard, Warrior, and second Rogue are able to kill it in one round, even with the Parry it wasn't enough (he's only got about 100hp).

We really need to build in a conversion of how to make creatures in the Beasteary turn into Solo monsters. For the most part I've decided to just double/tripple their HP and add +1 to their attributes and defenses, but even with that it's not a whole lot of extra help.

Any thoughts? 
i6 Ravenloft Conversion (4e) | Dungeon Master Encounter Sheet
My next session is against a deathknight, will let you know how it pans out. Out of curiosity what level is your players? Starting to worry it wont be the epic battle I had hoped for.
They were 12th level. I think that when they fight the reincarnated version that it will have double HP and +2 to all stats/defenses. That might do something... not sure though
Boss : +20 Speed, +50 Hit Points, Advantage vs 1 Save. Debuff spell once per day (color spray, darkness, hold person, silence, stinking cloud, web)

Not just stats though - use the environment or allies so the good guys can't all attack at once, so they wear the good guys down before they get to the boss.

Key things for D&D - Where is the character from and why do they do what they do? / Recurring NPCs - allies and enemies / Plot, World and Personal Events.