I decided to revisit the Celts, since it was quite a long time ago. They are just as fun as I remember them, you have necessitated a unique guerilla style warfare for them, since you struggle in a straightforward fight against the legions
At the start, I sent my settler south to Spain.
The Romans founded Lugundum, and I initiated war by taking it. The Roman legions went for Masilla, and my stack of gallic warriors shadowed them, picking off lone legions when possible, but never going close to the whole stack.
The guerilla warfare continued. I took Mediolanum, but evacuated Lugundum to them as well.
Their armies reached my productive capital, Lutetia, which I evacuated.
Vengeance come, as them sending their whole army north had left Rome exposed, which I took.
At that point, my army was exhausted, while much of their was intact. I sued for peace, hoping they would collapse. Alas, that would not be the case - they would fight on until the bitter end.
The world was upside down, but I thought I had weakened them and waited for the collapse.
In the meantime, I had sent two gallic warriors south. I waited until the Seleucids attacked Constantinople to get through, then I took Ephesus for getting Anatolia.
By 210 BC I had tired of Rome not collapsed, and attacked them again, retaking Lugundum and Lutetia, as they had sent their armies to conquer Greece.
They also had a large armie in Anatolia, threatening Ephesus, so I had to accept peace before they attacked.
The culture of Tarentum expanded, taking the food away from Rome, which was my primary settler factory, so I had to declare war again to get the pig.
Tarentum fell quickly. I had prioritized sailing, and started a galley early in Burdigala. Rome and the city in Spain built settlers for Germania, Brittania and Illyricum.
Again, I had to make peace quickly.
In 153 BC the first UHV condition was met.
Since the Romans showed no sign of collapsing, I renewed the war.
Salonika, and later Athens, fell, as they had only saved four legions there, sending the rest to Carthage and Syria.
Sicily was taken in an expedition.
My economy was in serious trouble. The only tech I had managed was sailing, so I couldn't let my cities build wealth. I gave away several cities to the Ptolemids, which provided a temporary relief. Begging, and threatening, the other civs gave the rest.
A Celtic expedition had to be sent to Syria to clean out the Romans.
They were entrenched deeply.
But everything fell.
Rome only had Carthage and Leptis Magna left. Still they persisted. I assembled a large expeditionary force, took Carthage, and with only a single city on the North African coast, they finally collapsed.
In 86 BC, I had also succeeded with the culture goal. The golden age had given me enough gold to do some research, so for the last UHV I would just have to press end of turn. Victory!
Stability might be bugged, they shouldn't have survived that long

Still, it was fun with the game gradually turning from me avoiding fighting at all cost to the hunter becoming the hunted. It would have been much easier if they had collapsed after the fall of Rome, so this was fun.
Thanks again for a fantastic mod!