Do NOT use First Strike. It is too slow. You can only use it in hyperscaling matchups like Veigar.

Use Arcane Comet against assassins. Grab dematerializer in your secondary for Q-W wave clear (2 melee, 1 ranged).

Use Dark Harvest as a standard loadout. Cheap Shot combined with Scorch makes trading very effective.

Focus on syncing your push with your lane opponent. If they basic attack minions, you basic attack. If they use abilities to shove, mimic that as well. You don't want to shove too far to get frozen, and you don't want the wave to crash into your tower because Xerath's turret CSing is NOT optimal.

Your standard trading pattern should be W into E. This will proc BOTH Cheap Shot AND Scorch. These trades are very effective and will quickly result in a Dark Harvest proc as well.

DO NOT USE E TO TRADE! Your E should only be used for disengage or for kill secures on ganks/team fights. If you use E in lane, you will not only consume too much mana, but you will be vulnerable to ganks/dives.

Now that first strike is almost never an option, EVERY game should start with Doran's Ring.

An ideal first back is 1300 gold for a Lost Chapter. However, if you are pushed out of lane prior to this, grab a Dark Seal & whatever Lost Chapter components you can afford.

Your mythic is determined by both ally & enemy team composition. Your standard mythic should be Luden's since Xerath performs very well with Magic Penetration. However, if the enemy team has multiple brawlers/tanks AND your team lacks good tank shred, you should opt into a Liandry's.

For boots, obviously, Sorcerer's Shoes are the standard in every game.

Your second full item is determined again by team compositions. If you were pushed into a Liandry's build, the enemy team is likely too tanky for flat magic penetration. Therefore, your 2nd item should be a Void Staff. If you have a Luden's, your standard followup should be a Shadowflame.

Regardless of composition, additional core items are Rabadon's Deathcap & Horizon Focus.

Substitute in a Zhonya's Hourglass as needed.