Replying despite my lack of Regrowth/Mod experience since I happened to see a set of videos addressing Leonard. Not only do you need sufficient altar power and the correct order of the numerous ingredients, your Skill Level for Witches' Brews need to be at 12. This is improved by grinding out custom brews on the cauldron; a read through of the book aptly titled "Witches' Brews" is recommended. You should be able to see your level by using a Seer's Stone. Once you take some time leveling up that skill, you should be all set to call the guy.
Side-note: In the video I watched, the player had a nightmarish time trying to fight down the boss and got stuck in the invulnerability phase. He would break the guy out of that state but whatever damage he did would already be regenerated. A quick glance through the comments says that you are expected to use the Witchery's brewing system to the utmost to shred through and neutralize most of the boss' abilities. I leave it to your research and theory-crafting for what would work in the end, since I'm not the one you should be asking for in depth info on this mod (lack of play experience). As a commentor put it: "This is basically a final exam to test whether you paid attention and learned the mod or just rushed through the quests and progression."
Hope that helps.
Extra Edit: I also remember that drawing Ritual Circles around the cauldron as described in the book will mitigate the altar power cost, if you want to soften the massive drain.
