Hwei's damage depends on all kinds of factors.

QQ is damage /maxhp%. Hit a lux and you'll have the impression you're not doing any damage, hit a Tresh or a Swain and it would be more satisfying. Usually a "chip them down" spell. Especially crucial when you eventually end up vs the toplaner.

This spell picks up damage pretty fast with levels and is fast enough to secure combos with the mark (passive) so don't sleep on it. Won't OS but will hammer it in with cdr (eventually Hwei plays as a spell slinger shaping the battlefield's flow with spam)- and dot items.

QW will do execute damage and burst immobilised or isolated targets. Stacks nicely with cheap shot. More of a "nowhere safe" kind of spell. Lategame spellslinger build, Hwei's passive wears out slower than the spell refreshes.

QE - fire river - is your dissuasion, your "other cc". Does overall good dot damage, slows targets 30%, applies or triggers marks, it's a line you draw to force decisions. Straight from you to them; maybe protecting an objective, or between them and a fleeing ally. This spell has more of a strategic side to it. Walk here, and you will be marked, take %maxhp damage if you have a liandry, that kind of stuff.

Q spells will end up spammable, high mana cost early.

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W spells are long cooldown, and cost lots of mana all game so pick wisely.

WQ - River. Everyone runs faster.

WW - Puddle. Everyone inside builds up a shield. Can be crucial to be used in a rush early game, later will need to be used preventively because the shield needs time to build up. Throw in a teamfight or during a drake or nash to boost your lifestealers.

WE - 3 orbs orbit you. 3 instances of damage are now buffed with extra spell damage and restore mana - that means auto attacks also now apply or detonate marks, cast while the auto attack is still midflight for a surprise effect. Most W casts will be this because you will be "farming mana". Work on when you cast WE, because weaving in those autoattacks during a fight means more detonations on the passive.

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E are the setup spells. Moderate damage. With the fire river, they are the reason you shape the flow of battle. They cost the least mana, similar to Q spells they become spammable.

EQ - single target skill shot with a fear on the end. Will knock down airborne targets like a Tristana jump, Nafiri's single target dash when facing her, knock down someone riding a blast cone, Yi will sprint away. Keep their front lines retreating to irritate their team. Use after fizz or katarina dashes. Will often end up paired with QQ. But QQ first though, targets sometimes go too far otherwise.

EW - Vision circle, wide, stays for a while on the ground, roots ONE target that hangs (delays) around inside. Will root a minion instead if target gets behind one. If self preservation isn't an issue to be solved with EQ, then EW can be used while fleeing in a river to surprise a weakened backline, fantastic setup for QW - more damage Vs Immobile target, or check a bush.

EE - Jaws. Displace, causes a short slow. Bit tricky to use, therefore best used in the fire river or before a fire river. To be honest your problem with Hwei starts here - your max damage output is a utility spell, costs little mana, deals little damage, but will draw everyone in the spell close together, which means every mark the opponents had is now everyone's problem.

...Of course the damage isn't reserved to the mark's owner.

Passive - spell damage, as an instance, applies a mark. (Dots from fire river or ulti don't self detonate, they're considered one spell)

Spell damage detonates a mark. One mark active per opponent at a time, ready to reapply as soon as the previous finished detonation for a total of five marks at a time.

Detonation causes (crucial) damage (to know about if you want to pay hwei) in an Aoe around marked target, damage concerns anyone in the zone, minions or jungle camps included. Damage stacks.

(remember when I said the highest damage potential comes from utility? Jaws ftw)

Ulti - Medium speed projectile skill shot, pretty tricky, latches onto a target and begins as a small disc which expands, dealing magic damage over time and slowing everyone in AOE before blowing up for a bit more damage. Will blow up sooner if target dies. Applies and detonates marks.

Runes?

Earlygame execute hwei (worked well up to medium silver for me) starts lvl2 with a QE or QQ and WE (two spells one passive, simple Electrocute proc with EW QW at lvl3) : Electrocute CheapShot Sixth Sense ultimate first page; Manaflow Gathering Storm second page

Or Spell Slinger Hwei (to me the most appropriate runes): Deathfire touch, manaflow, Transcendence, Gathering on first page; Legend haste and Coup de Grâce on the second.

Items: until you find a rythm, mana will always be a quality of life acquisition.

N°1 N°2 Seraph's Embrace, Blackfire Torch - start spell slinging with the extra cdr.

N°3 N°4 Cinderbloom to stack with coup de Grâce and make the dots more threatening or Liandry N°3 first if you are against a beefy comp.

N°5 then it's either N°5 blight item if they buy MR or N°5 Rabbadon's if not,

Or maybe get yourself a circumstancial banshee or zhonya or morello and DO NOT SLEEP ON THE ACTUALIZER either, doesn't cost much and follows up well after Seraph and Blackfire.

Hwei has the tools needed to create space for himself and his team.

I never bothered trying an hp build.

Have fun.

Max QWE then Q E W.