Watch Ninu's gameplay. He was a pro DPS player for Navi.
https://www.youtube.com/watch?v=9zRoLQqvfsE
https://www.youtube.com/watch?v=1j7MbUbJPN0
https://www.youtube.com/watch?v=1ZJ71Py_M64
https://www.youtube.com/watch?v=egLt1iduEcM

Best: Jaguar Falls, Brightmarsh, Ice Mines

Good: Stone Keep, Serpent Beach, Splitstone Quarry, Ascension's Peak, Warder's Gate

Meh: Bazaar, Shattered Desert

Worst: Frozen Guard, Timber Mill, Frog Isle, Fish Market

Willo likes maps with small closed areas, high ground and rooftops to Ult on.

She's a great early-pick on Jaguar Falls.

Brightmarsh - Go "lip" (the roof of the mid point building) in the mid. Then, garden (that tree area high ground). During the payload phase, zone out the greenhouse area.

Stone Keep - Try to control Keep for the mid fight and catwalk/bridges high ground during the payload phase

Ascension Peak - Backline DPS have crazy sightlines on this map, you gotta watch out. But you also get crazy angles to Ult from. Control the stairs in the mid fight. And high ground during payload phase.

Warder's Gate - During the payload phase, she can't fly up to the area near the enemy's spawn (sadge). But there's 3 spammable chokepoints there, so it evens out I guess.

Serpent Beach - Gotta watch out for those DPS sightlines but there's insane verticality and high ground on this map.

Splitstone Quarry - You can play high ground or quarry, whatever's safer. Maybe quarry if they have long-range DPS. Or high ground if they have short-range champs. Lava room is risky but if youre confident you can use flutter to fly up to the 2nd floor to juke an enemy.

Shattered Desert - has some spammable chokes, but it's easy for flanks to reach her but there's no high ground at all.

Bazaar - second half is good for Willo, but the mid point has lots of sightlines for DD's. She can play on these 2 maps if her matchups are fine.

The map contextualizes matchups. For example, Willo is weak against DD's like Viktor, but Jaguar Falls is Willo's best map and Viktor's worst, so the matchup may favor her on that map.

For Onslaught: Primal Court and Foreman's Rise are great. Magistrates Archive is good. Snowfall Junction is okay. Trade District and Marauder's Port, ehh...

Willo's role is backline DPS or an off-lane DPS (though usually Backline)

High Ground is very important. It offers a lot of benefits:

It gives you cover vertically and makes it harder for enemies to hit you

You get better sightlines

It's easier to hit your blaster shots from above than at equal level

It's harder for flanks to reach you. Many flanks have no vertical mobility at all. And the ones that do, you're forcing them to spend a cooldown to reach you.

It gives you some extra altitude for using your Ultimate

Flutter lets you rotate to high ground, so there's no excuse to spend the whole game on low ground. Sometimes you drop, by accident or by valid reason, but try to Flutter up immediately so you don't waste time and you get Flutter back soon.

Supports: She counters them. Dead Zone blocks their healing and her Ult can kill them.

Tanks: She's great against slow and passive tanks that want to hold one area for an extended period of time. However, high mobility off-tanks like Raum, Ash, and Ruckus can pair up with a flank and dive her. Atlas is 50/50; he is a slow passive tank, but he can land charged shots on Willo in the air.

DPS:

She's good against close range DPS with limited mobility (i.e. Grohk, Moji, Tyra, Vivian, etc). Yes, I said Vivian. Keep your distance, use high ground and try not to turn around corners alone because they can ambush you.

Her weaknesses are: Mid-long range DD's: Strix, Kinessa, Viktor, Cassie, Lian, Shalin Flanks: Maeve, Andro, Lex, Buck, etc.

Willo is not a great 1v1 duelist against DPS. But she makes up for it with all the utility, zoning and cleave she offers in the team fight.

Primary Weapon is AOE (blaster). When at equal level to an enemy, try to shoot at their feet.

Flutter is your only defensive ability (it also gives you 25% DR with the card) so try to save it for 1v1 duels if you can, or escaping Ultimates. You can fly over an enemy's head at close range to juke them.

Seedling can zone small areas. Zoning points of entry like doorways and stairs prevents enemies from entering and exiting.

Dead Zone is a kill confirmation tool. You're not just doing it for zoning alone like with Seedling. You're marking an enemy for death. So try to follow up with damage on whoever you're Dead Zoning.

A common mistake I see on Willo is that people tunnel vision on the point and use both of their zoning cooldowns on point. Zoning point can be good, especially during overtime, but expand beyond that. You are a wise tactician that can exert your influence on any area of the map. Your zoning can stop an enemy backline Damage from peeking. Could stop an enemy flank/aggro DPS from entering the flank route they want. Or stop the enemy off-tank from pushing.

Let's talk about one of the best and most underrated DPS Ults in the game.

It's a fast charging Ult. You can have it every fight or two. Don't hold onto it for too long.

It exposes enemies who are hiding behind walls and shields

It's free wallhacks (like Cassie Ult). You see where all the enemies are and call it out to your team.

It gives you access to the enemy support, who is normally hidden where most other Damages can't see them. You either kill them in 5 shots, or at least force them to rotate into a bad position.

It lets you win 1v1 duels against most champs (which is otherwise's Willo's big weakness)

It gives her CC Immunity, some outplay potential there

It's a rotating tool. You can flank to the enemy's side of the map. Or safely escape to your side of the map instead of dying. You can even fly around the edges of maps with cliffs.

I know you're thinking "Really good hitscan players will easily just shoot her in the air right?". Nope.

You don't activate it in their LoS. You find an off-angle or some moment when they're distracted.

Activate your Ult nearby rooftops and tall buildings to use by cover. Willo can take a shot or 2, and then dip back down to cover. Do not activate this Ult near the open. Be by cover.

Draft her well and this is less of a problem. Sure, it's a bad Ult on an open map against long range champs, but smart people don't draft her in those situations, so it's not an issue.

Who can hit her in Ultimate? Lian Presence, Atlas charged shots, Kinessa and Strix. Andro can fly up. Cassie/Shalin maybe, but not consistently. Those are a few champs, but otherwise the vast majority of champs in the game can't contest you much. Keep in mind that most hitscans have effective ranges, and if you fly up far enough away from them, you out-damage them.

Tip: Use your Flutter to get some height before using Fae Flight. You'll get it back after the Ult anyway.