I played with Barghest in the upper room.
Then i had some Flesh Moulders in the left room with the hidden key token and had Shadow Dragons in the bottomright room with the other hidder key token.
In the closed room i had 1 master zombie and 1 minion zombie.
In the bottom right room i placed the Dragons on the far side of the room, so that attacking them would take to much time and effort.
The fleshmoulders i spread in a line of 4 blocking the way to the token.
They had to rush to the tokens, because i was punishing the cardinal hard.
This meant that my dragons were unharmed untill the lockeddoor opened.
As soon as the locked door opened I dashed my dragons straight into the same room as the cardinal.
Blocking the path for him with my massive base. I also sent the remaining fleshmoulders that way, healing the dragons every chance they got, stalling the heroes from killing the dragon to unblock the path for the cardinal.
I have to say i was extremely lucky with some terrible attribute tests that the heroes rolled for the altar.
But in the end i easily killed the cardinal this way and even managed to drop a hero or 2.
Key to this mission is to save your 2 monstergroups until the locked door is opened, then make a run for it to block every single possible path the heroes have for getting to the cardinal. Pittraps and Tripwires also come in handy because of this.
And save your Dark Charm in case you need it to move a hero out of the way so your dragon can expand into a good spot.