Sions with the Trainer traits will also help (if they can get them; it's been a while since I bothered with Sions), if you're playing an Empire, since Henchmen are only available to the Race of Men (default faction Altar).
Increasing quest frequency in game setup or spamming quest maps in game will also help, as will davrovana's suggestion of increasing the world difficulty. If you didn't take the faction trait that grants quest maps, Altar might have them for sale in its shop, if they've researched the tech for it (as might any custom faction with the right trait).
A great deal of patience and farming AI armies can also do it.
Not clearing out things that spawn wandering monsters and then killing whatever does spawn will help (generally speaking, this is worth more experience than AI armies are, even if the spawn is a Bandit and his pet wolf, but you need to keep the AI from clearing it or you just lost your experience farm, and I would tend to recommend that you not leave a whole lot of such things around if they're within your area of influence, because the lair guards will tend to wander off, making it that much easier for a random AI champion to raze the lair).