Fortified pikemen on a hill/forest: 27.5. Or pikemen with flanking bonuses: 21.5-23. Not enough to easily dominate, but pikemen are easier to build/buy.
Problem with this is that units that are supposed to be hard counters really fall behind the units they counter when you start getting promotions/generals stacked up. A 22 strength elephant with a great general bonus is strength 27.5. A strength 10 pikeman with a great general and a 100% bonus against said elephant has strength 22.5, or at a 22% strength disadvantage. Add 15% for discipline, or a shock/drill promotion to both sides, and the elephant keeps getting double the bonus the pike gets. Since elephants promote from horsemen, it's not unreasonable to have shock/drill III by that point, and perhaps charge, blitz, or other high-end promotions.
So say both sides have drill III, a great general, and 15% from discipline. Sure one side or the other will likely have a promo advantage but we'll just assume here since the elephant is just as likely (or more likely, really) to have more promos than fewer. In that case, we have:
Pike: 10 str + 100% from vs. mounted + 15% from discipline + 25% from great general + 25% fortification + 25% rough + 75% drill III = 36.5 strength
Elephant: 22 str + 15% from discipline + 25% from great general + 75% from drill III = 47.3 strength
Now remember, with no bonuses, the pike only has a 10% strength disadvantage on the elephant. But, once you add in the modifiers that you're likely to be seeing by this point in the game, even if the elephant is attacking onto a fortified, rough terrain pike, it has a 30% strength advantage. And yes, pikes are easier to get than elephants, but with 1upt, greater numbers of weaker units is almost always a sure-loss strategy. Even if the elephants get ganged up on, that would result in the pikes forfeit the 50% bonus from fortification and rough terrain defense in the above calculation, making it 31.5 strength vs 47.3 strength, or a 50% strength deficit compared to the unit the pike's supposed to counter.
This also doesn't take into account any ranged units that would factor in. If you're not happy attacking into a fortified rough terrain counter unit and still having a 30% strength advantage and instead decide to let each side's ranged take shots, any archers/crossbows/catapults etc. on either side are going to be dealing way more damage to the 10-strength pikes (even if they're getting fortification bonuses that the elephants can't) way faster than to the 22-strength elephant. So the pikes can't really play a holdout game either.
IMO, this isn't as much a problem with elephants as it is with game mechanics. With additive % bonuses and low strength/high bonus counter units, the inevitable outcome is that as bonuses mount, the counter units will get crushed. There are a few ways to fix that, but all of them suck in some way.
