I camped the castle walls for a couple days and had zero luck. Then after talking to another druid, I went back to Velk's and got the heart in less than 3 hrs, after clearing room 8 times.
Velketor's Experiment (green to lvl 75) spawns in the room past Velk's room. (Velk's room is in the castle basement). Continue up the stairs past Velk's room and it dead ends into another room with 6 mobs (gargoyle's and golems). That's your camp. Bledrek and Bled also spawn in this room (out of 8 clearings, they spawned twice).
Mobs repop about every 20 mins.