I'm not saying it's a foolproof tactic; it relies on luck, but the entire game is based on luck. Regarding your idea, I think it's much more difficult to implement in practice. First, mentoring opportunities with Necropolis heroes are rare; I've only encountered them a couple of times. Second, finding Wingael himself is practically impossible; it's already a stroke of luck if you manage to find any Elf hero. The problem is that I used to think that the heroes in the tavern depended on the castle, but I was wrong. Also, even with mentoring, Wingael's level might be too low. So, it's much easier to find any Elf hero early on and find a Tome of Darkness to level him up a bit – and then go into battle. And even that depends on finding an Elf early on; by the end of the game, he's no longer useful. Okay, this is indeed a luck-based tactic. However, you can start with an Elf, find a Tome of Darkness, and it will be easier, and you might not even need Necropolis heroes – that's why I'm describing this tactic in the Sylvian thread and not somewhere in the Necropolis thread.
For Necropolis, a much more effective and predictable tactic is Necropolis + Haven + Knight. You can train a lot of Banshees, Messengers, or even Liches, depending on your preference.