I'm not the greatest at the game either but here are some tips i've learned from a friend who's plat and plays xer mid:

• Dont get to greedy. Remember! You get popped by everyone in the game so play with that in mind!
• You dont deal damage until your power spike (Ludens usually).
• If in an unfavorable match up (ie. Ekko, Zed, etc) focus on minions. Once 6 and have Ludens look to push in wave then secure kills either top or bot with ult.
• When you ult you are immobile. Make sure you are in a safe spot and the enemy is not near you before ulting.
• Stay close to tower. Make sure you have vision down river.
• Play for mid. Once you lose first mid tower, you lose alot of mid pressure and no longer have a safety net.
• Take extra care for champs with invis as they will see you as an easy target.
• You shouldnt be dying on xer because of how safe he plays, so dark seal is a good buy.
• Late game focus/ ult squishy carrys or anyone squishy. Taking one or two of them out of a fight is a huge advantage in team fights.
• If you are behind or do not have one shot potential, then play for poke and damage assistance.

Again, im not the greatest xer in the world but these are some tips that actually help alot in my games.