though I find the Celts to be quite easy, they only have to push the Romans out of Britain, which isn't hard.

Then fight off the emerging Romano-British, also easy , considered the AI's siege capabilities - though harder if you decide to meet them in the open.

And after that is done, you only need...about 2 more main land - weakend Romans cities.

So to make a long story short, try Sassanids, challenge if you turtle, and they also have nice objectives and units.
A good all round faction, like hte SE was in 1.3