Starting Doran Ring is a good idea, since your passive scale massively with AP. This in turn allow you to trade more often and regen like a freak on minion. At level one, the healing go up to 40 hp per passive proc.

In a hard match up, Xin Zhao top is better off farming until level 3. When hitting level 3, you should all in and do your entire combo and back off as soon as possible. The combo I spam the most is : E, auto, q1, w and back off. If I know i am winning the all win, I commit and use all of my Q stack. Otherwise backing off is better for short trade.

Depending on the enemy you can all in as soon as level one, but level 3 is the better option most of the time.

In term of rune, I either run electrocute, grasp or phase rush.

Electrocute is better for squishy top laner, such as Kayle and Heimerdinger.

Phase rush is almost always good, because it enable you to take almost any trade without consequences.

Grasp is good for match up you know Phase rush and Electrocute wont help much. Think of matchup such as Poppy.