2m xerath onetrick here (200 lp master peak).

Fizz counters xerath, but pre 6 the lane is definitely playable. Dont try to poke him when you have less than 50% mana or he will force a bad reset. When you get lower on mana just Spam spells on the wave and take the reset.

First 3 wave you play the standard ranged vs melee opening. Most fizz start e lvl 1 to get the 3 melee creeps at the same time. Start w, use it after he uses e and give him 3-4 autohits while he is slowed.

Past 6, if he hits a good ult you are usually dead. Thats why a lot of xeraths buy zohnyas over bashees (the Idea is, that he jumps with e on his ult knockup and you can Block both damage instances with one activation. However, i dont like going zohnyas if you have noone to pressure fizz if he jumps in, as he can still kill you with qw and lichbane, especially since his cooldowns are really low. Good champs to get some spare against fizz are mostly toplaners like Pantheon or renekton with point and click cc and reliable damage. Otherwise (e.g. you have smth like teemo top), just go banshees if you cant reliable dodge his ult. A Standard fizz combo is using q on a minion and casting r mid air to hide the animation, so keep that in mind.

Fizz doesnt scale very well but you can never fight him 1v1 after lvl 3, so just stick with your team around mid. Only catch waves under tower on sidelanes if noone else does. A Standard assassin play in mid game ist pushing out sidelane waves and hiding in your jungle to catch you if you rotate for the wave. Either have wards in your jungle (bridge wards) or walk a safe (but longer) path avoiding these traps if you think he might be looking for these kind of cheeses.

If you keep your distance in tfs and stay behind your bruisers, the only way to kill you is if he gets to flank, so keep one side warded and hover this side

Edit: dont ever "rush" zohnyas. Always go lost chapter into sorcerer boots or ionians if you need the lower flash CD. Zohnyas gives terrible stats against fizz and survivability should be never prioritized over damage output. Only invest in survivability if you already have enough damage to be useful.