Not sure what the minimum level would be - I took him on at 53rd level and didn't have too much trouble, so long as I didn't mind losing some troops. Fighting him no-loss took a bit more effort, paying attention to my tactics and using time back once.
Keep in mind that he's not a boss, like the Driller. He's an enemy hero with a very strong orc army (including plenty of dangerous shamans). You can use your dragon, and he'll have spells (he mainly used phantom when I fought him, but he probably has others).
In a future game, I'd be reluctant to take him on at a lower level than the mid-thirties. You can always give it a shot sooner - just be sure to save first.
