Druids with the Order religion can also gain Command 4, as it only requires Order religion + Channeling 3, and disciple unit class.
None of the high base strength units (riders of apocalypse, dragons, avatars) have the hero promotion.
Ironically, I was able to add enough equipment to my baby Acheron (19 strength) to make him truly powerful (33 strength) ... with Netherblade, Orthus Axe, Athame, Staff of Souls, and War ... meanwhile Mardero had command 3 and crown of command.
Just for fun I mopped up aka declared war on the last remaining civilization ... with ONLY acheron ... and with 33 str plus drill 4 (eventually) he finally felt like a respectable monster ... like he finally reached the dragon-hood he truly deserved. Tis unfortunate that the AI gained him a few extraneous promotions beforehand however, So I had to level him a bit more than normal. Still, the tech lead really helped as well. So Acheron with 33 strength finally felt like a dragon among archers, warriors, freaks and puppets. I might should try out the Monstrous Creature mod, but I digress.
I've found diseased corpses and beasts of agares are usually the best tools for killing the Riders of the Appocalypse. They have access to formation 2 (120% total vs mounted) and incidentally also have access to shock 2 (120% total vs melee) so I took one of each. Unfortunalely a dwarven slinger killed my melee slayer, because I fielded him alone vs the dwarves. However my Beastmaster defended against about 20-30 dwarven warriors in one turn all by himself. (they were all RoF'ed warriors, and he was protecting a large stack of cats with Drill 2, which might have slightly damaged the incoming warriors.
It would be nice if there was some expensive ritual to grant Baron the hero promotion ... perhaps by sacrificing a religious hero or somesuch ... or just sacrificing something fairly significant but still not too expensive. After all, you had to complete the ritual first, and you'll have to wait for that experience to slowly accrue.