If you are not able to attack a champion, restore mana by attacking minions but try to attack the minions with more health to avoid stealing farm from your carry. Make sure to utilize Xerath's passive (restoring mana on basic attacks once every 12 seconds, double against Champions) by attacking enemy champions with basic attacks whenever your passive is available and you have the opportunity. If you can avoid leona e you should be fine, zyra doesnt even need to hit her spells, 2 aa's of her plant with spellthief proc and 1/4 of your health is gone. The Best Xerath build guide by OP.GG - Providing Xerath build with the highest win and pick rate. Continuously poking down the enemy ADC will also do a good job of keeping them off farm (since you can hit them at such a far range) and force them to back much more frequently causing them to also lose XP as well as gold. MF+Zyra is the clear winner here i think. Remember to focus your poke on the ADC as much as possible and try to sieze opportunities where you can hit both champions at once to conserve mana and maximize gold income. I was Diamond 2 back 3 months (quitted playing and lost LP, Plat 1 right now with Dia 2 MMR). This will grant you gold and give your lane and Jungler better opportunities to secure kills during ganks or just in general. Selling Diamond 2 Xerath only LoL Account INCLUDING XERATH BOT/SCRIPT This is a SPECIAL League of Legends Account which can get you to Diamond 1+.
Your main focus is to poke down the enemy ADC as much as you can.