How are you doing this? Something has to be really off, or there is something that I am absolutely not understanding. I'm lucky if I can start with two promotions. If I am this far out of the game, then I may as well just resign now.
About how op these promos are? These hero promos are just incredibly op, so about them there is nothing I can do. About the free 4 promos? That's easy: Tatoo Hut gives
Tatoos,
Posion Tips from the Posion building I forgot the name,
Guerilla I from Shamanic Building and
Desert I from Mesa Verde. About starting with 20+XP? My capital has everything I can build that gives XP with the techs I know, plus some special buildings from animals and things like that and also the wonders you can check in the Top 5 Cities Screen. Chariot Archers start with 26XP, plus free promos as Archer and as Mounted.
Total War with you is a choice I didn't make. There is much to gain with you alive. Also, I'm keeping what I said before. There are only 3 cities which aren't supposed to be where they are. The rest I don't bother, and this includes your capital.
As a guess, he received a Great General and used it to attach to a specific new unit. Doing this gives the unit so attached +20 XP, and if it was already receiving several XP (from Buildings, Wonders, etc) the unit can begin at a fairly high level. If the GG was attached to an already Hero unit (the unique ones that are unlocked by specific Culture Wonders), it can have even more promotions. It sounds like he did that. It also sounds like he gains 4 free promotions, probably from a combination of Leader Traits and Buildings.
You may still be able to do well, especially if you can crank out more units per turn than he can. Sometimes quantity can overwhelm quality. Also, you will hold out longer than an AI would, so retiring at this point would actually help him. I'm guessing that you don't really want to help him. I could be wrong.
For the first part I have already answered above, and no general was used on any of my heroes. For the second part, no he can't make more units. He doesn't have the money. You see, he is still at 100%

, but he is already losing some dozens of gold per turn. He'll be forced to reduce his spending soon, and his Technological situation isn't doing great. But he has a sizeable army, with some resources to get some nice specialized units of this era. Total War with him is costy, and quick annexation is probably undoable. With Revolution On, I can't control portions of his empire, there is no city he has which has more of my culture then of his own. Joseph's ex-capital swallowed like 2000

from me in bribes already.
Also even if he wanted, 6-7 cities can't produce more then 21 cities. If I were solid first GNP for ages, with a certain degree of Technological Advance over the second best, this war wouldn't be as light as I have been making it. I can say that aside from losing Mayta Capac, everything is going according to plan.