Rider Magic is only stronger if the Rider draws energy from there Dragon. But even then it’s possible to for an Elf to overpower a Rider. Vanir did so to Eragon

We saw Eragon beat Murtagh with the help of 13 elves. So logically it stands to reason that if Murtagh had to fight an army of Elves ( tens if not hundreds of thousands of them) he would not stand a chance.

Evles are already far stronger than humans and they heavily outnumber Murtagh. Strategically it wasn’t a smart move to send Murtagh to fight them

And the Elves has already captured the City of Cenuon by this point so the idea that they didn’t know how Galbatorix would react if they attacked his city is false....also I’m not sure what that has to do with the topic at hand