Just wanna say I finally beat this nightmare of a mission. I was never able to beat it “normally”, always had to use the cheese strat of just rushing the objective ASAP and getting it out that way. No matter what “normal” strategy I used, the AI would just overwhelm me with an endless stream of units.
This time though, I tried doing the Roman strat of just spamming towers, and…surprisingly, it worked. With a handful of Guard Towers, alongside Chariots (since the AI kept making lots of Priests), I just held off until eventually the AI just stopped attacking (I guess they ran out of resources?) and then I just slowly cleaned them up with Chariots from there.
No other mission in the entire official campaigns gave me trouble, but for some reason this one seemed utterly impossible.