[Baalz]

Admittedly I haven't been on the receiving end of an elephant rush orchestrated by you, that might be fun to try as I have beat back other rushes playing Ulm. Ignoring castle-recruitment constraints for a second, for the price of 3 elephants you get 2 smiths plus change. Obviously this isn't scalable because you can only recruit one smith per castle, but still given Ulm's typical early gold advantage and urgent castle building it's not unreasonable to expect to be able to field 10-12 smiths by the time you're looking at a 20 strong elephant force threatening you - and without permanently crippling your research curve. Assuming you marched a force as you describe into my territory at the end of year one I'd counter by trying to catch you as you attacked (shouldn't be hard) so you're running into about 40 PD backed by the dozen or so smiths leading whatever of the halberd wielding infantry I could scrape up over a couple turns from my multiple castles and prod-3 edge (likely in the 100-120 man range). Your initial elephant charge will be broken by 7 or 8 smiths spamming Bonds of Fire. It won't stop them all but they'll trickle in several at a time rather than in one overwhelming mass as even the ones not hit will have to run around those that are. While causing some damage, they'll quickly be surrounded and hacked to pieces by halberds. Meanwhile my remaining smiths begin raining magma bolts down into the elephants strung out by the fire bonds while my black plate keeps friendly fire from being too much of an issue. All the while your astrologers are popping 4-5 of my infantry per turn out of probably close to 200. By the end of the battle you've killed mostly my PD (which were in the front) and are out close to 2000 gold of elephants.

Would it work that well in practice? Perhaps, that's debatable (or better yet testable), but you'd have a hard time arguing that it'd have no chance. My point is that Ulm isn't helpless, you just have to use what you have at your disposal. Cheap troops, cheap mages, lots of gold/castles.