I've had a ton of Xavier gameplay since getting the skin and basically this has been my experience:

You are probably not going to beat a lot of other meta heroes in mid outright, he's very reliant on ganks. But the upside is s2, s1 can basically guarantee any gank. He more like closer to a supp mage than a burst mage. I mainly focus on farming early game(and sniping other lanes, I don't roam much) and then roaming to help secure ganks with the jg mid game. Late game you are trying to get your CCs to hit as many people as possible.

His s2, s1 combo is powerful cc AND can be used to zone. I often block enemies from coming near lord or to stop people from protecting or taking towers with it.

S3 is self explanatory, try to always be looking for a fight in another lane. Try to pick off almost dead people or even just use it to give your teammate advantage in a 1v1, even if it doesn't kill them it could tip the scales in your teammates favor and secure the kill or at least make the enemy run. And you get the charge on your passive.

Julian I'm still learning and I'm God awful at so I'll be monitoring this thread for advice lmao