Overview
Xavier is good mage for team fights, you can stun enemy heroes with a skill 1 and 2 combo, and he is more like a finisher if an enemy escaped while his hp is low. You should play more safe with Xavier, even you have the least gold in your team.
Strengths
Try to farm every wave in your lane and not lose the tower, keep rotating to other lanes bcs Xavier is good hero to help in fights, even in early game.
Weaknesses
Xavier's counters are heroes with high mobility or fighters.
Mage
Best equipment to use.
CD reduction
Since Xavier's skills have high CD, the best build is CD reduction.
Battle Spell
For me flicker is the best spell for Xavier, you can escape from combos easly.
How to be more useful in teamfights
Stay behind your team so you have more range to do your skills, focus on your map so you can finish an enemy with your ult if they escaped, don't waste your skills.
Best With Miya
100% co-op win rate
Xavier can burst and kill squishy heroes easly.
Watch Out For Aamon
80% counter win rate
Aamon can easly get to Xavier since he have high mobility.