Lulu's strength comes from her large amount of utility and high base damage from her skills. Pix, Faerie Companion encourages players to harass more with auto-attacks as it scales quite well, and her Glitterlance, Whimsy, Help, Pix!, and Wild Growth, have both offensive and defensive properties. Thus, decision making with her skills is vital to creating openings for her team to follow up on.
Glitterlance has a moderate mana cost at early ranks. Use it sparingly to poke, as it can quickly drain your mana.
You can use Help, Pix! on allies, enemies, or minions in order to greatly extend the range on Glitterlance and make an unexpected poke at an enemy champion.
However, for better mana efficiency, auto-attack a few times before casting Help, Pix! on an enemy. As they try to retaliate or escape, use Glitterlance on them to disengage or continue the attack.
Lulu excels at supporting initiation: attach Pix to a tank, then Whimsy them to allow them to move into range of the enemy quickly and fire off a Glitterlance to slow much of the enemy team.
use Wild Growth then and you will disable the entire enemy team for a couple of seconds.
A single rank Whimsy makes Lulu quite mobile, allowing for ganks or wards depending on her build.
Use Help, Pix! on enemies who can use stealth like Akali, Vayne or Twitch to keep them revealed and sabotage one of their biggest strengths.
Use Help, Pix! on high attack speed allied champions such as Master Yi, Fiora, or Evelynn as they will trigger your passive damage more often and since they are squishy most of time, they will benefit from the shield.
Wild Growth has many uses; it can enhance an already disruptive tank with its knockup and slow field, make a bruiser virtually impossible to peel off an enemy carry, or even protect a carry of your own. In lane, it can make ganks from champions with a leap or dash ability more potent, or to help escape enemy ganks. In general, it should be used early and often as its cooldown is far shorter than most ultimates.
If timed correctly, Wild Growth can be used to interrupt dashes or delay escapes long enough for your allies to finish the job.
Wild Growth does not grab turret or minion aggro when knocking up enemy champions as it does not do damage. Thus it can be safely used within turret range.
Wild Growth isn't affected by healing reduction because it increases health, not heals it.
In particular, Wild Growth synergizes exceptionally well with Yasuo, as it can knock up several enemies in a large area for him to use Last Breath on, along with a cooldown low enough to use it again in another fight.
Use Whimsy to help allies escape dangerous situations. Casting Whimsy on them or on the enemy will make for an easier getaway.
At 40% cooldown, you can constantly keep Help, Pix! on an ally.
Cooldown reduction in general is exceptional on Lulu as it allows her to keep constant CC on enemies in teamfights.
Lulu can easily support other champions from the back, making aura items a viable choice.
Zeke's Herald gives you a bit of health, a good amount of Cooldown Reduction and also grants your AD allies bonus attack damage and life steal.
As an AP support, Lulu can pick up Spellthief's Edge as a good starting item since her kit encourages her to auto-attack and harass. The active of its final upgrade, Frost Queen's Claim, can allow you to catch out enemies from a longer distance.
Liandry's Torment makes her more resilient, boost all her abilities, and has great synergy with her massive amount of CC, making it a good first damage item.
While interesting to some, this build is sub-optimal as Lulu was created with a kit to support rather than to be a damage carry. A potential use of this build would be if your team has a substantial lead or is lacking a proper carry, such as ARAMs.
On the other hand, mixing AP with attack speed and on-hit items adds a lot of damage per second and utility in her kit in conjunction to Pix, Faerie Companion will max the fullest potential in her kit. Be sure to mix and match attack speed and AP when appropriate, prioritizing with AP first.
Mercury's Treads reduces both the duration of her disables, as well as her damage output, making it an ideal item.
Lulu does not have a very high natural damage output, so when alone or focused, she'll die quickly without much retaliation.
However, Lulu has a much higher damage output than most other support champions along with her terrifying amount of CC. Don't ignore her in team fights.
Despite Lulu's strong crowd control, poke, and peeling power, she does not have any form of heals to keep the AD carry healthy except for her ultimate, and her abilities are mana intensive. Therefore, support champions with healing like Sona, Soraka, and Nami can stay in lane longer.
Lulu's true power comes from the CC she can apply, and as such, is extremely dependent on her teammates; her damage doesn't scale well into the late game, due to her medium ratios, but remains a powerful disabler.
By late game Whimsy can give a very high speed boost depending on her build, and can be used quite often in conjunction with CDR. Beware if she uses it on a power initiator as it could mean an easy engagement.
In lane, Lulu will often harass heavily with her Glitterlance and her passive. Unlike many piercing skillshots, standing behind minions will not mitigate the damage, so moving erratically can help mitigate damage taken.
Lulu has no consistent heal, so poking at her or her lane partner will whittle down their health or waste her mana trying to shield them.
Lulu has mediocre auto-attack damage (44.4) as her passive gives her more damage on attacks and it quickly scales as she levels up. Do not be afraid to exchange auto-attacks early with Lulu as champions often have more magic resist than armor early on. (Pix will barely deal about 5.5 damage at level one)
Be wary that Pix will become a threat if Lulu has built a lot of attack speed. The damage scales quickly, and getting hit by all three bolts lets her outdamage carries during the laning phase. By level 17 Pix adds up to 105 magic damage per auto (not accounting for AP), and becomes even deadlier when used on a carry with higher attack speed. By this time champions don't have much magic resist unless they have itemized for it, making it deceptively powerful.
Wait for Lulu to use Whimsy before trying to use any channeled abilities or heading into the fight. This leaves her without the polymorph or speed buff, and allows your team to fight without disruption. It has a fairly high cooldown, especially at earlier ranks, meaning you have a good amount of time to fight before Lulu can use it again.
When she and her lane partner hit level 6 it can be very hard to beat them in a 2v2 situation as Wild Growth delivers a devastating disable and heals her target for a significant amount.
Like many ultimates on supports, Wild Growth has a somewhat high mana cost and can be rendered unusable if you manage to harass her into draining her mana pool.
Lulu is an excellent baiter due to Wild Growth; be very careful about diving or committing to kill her if she has the ability up.
Watch out for Help, Pix! as Glitterlance's second bolt will fire from Pix's position, meaning that Lulu can pick up an unexpected kill on you or a nearby ally if she tags you or a nearby champion/minion with it.
Despite Lulu being squishy, she is extremely hard to catch, being able to escape with Whimsy, slow you with Glitterlance, prevent some damage with Help, Pix! and if you get too close she will send you flying with Wild Growth while the slowing field will keep you from catching up.
Try to get Lulu to waste her ultimate without her team initiating. If Lulu uses Wild Growth in conjunction with a great initiating champion such as Malphite or Maokai, your team will be locked up for a few seconds.
If she does use it, be careful of what you do next.
While the knock up, health, and slow are disruptive, they don't hurt your team much after the initial knock up, so staying and fighting may be a good option if you can take out the champion before Lulu can shield them or give them the ability power and speed buff from Whimsy.
If you have to run, beware of the slows from Wild Growth and Glitterlance. If you get slowed by both of them, a polymorph from Whimsy may take you out if Lulu's team is nearby. Also keep in mind that Help, Pix! extends Glitterlance's range, making it a very effective chasing tool if she attaches it to you or a nearby champion/minion.
Finally, remember that Help, Pix! reveals a tagged enemy champion until Pix leaves, making it impossible to juke through bushes or past corners for the duration of the debuff.
Lulu is one of the few champions that has strong flexibility in her item builds and kit in a similar vein to Kennen and Teemo. Pay attention to her build and build accordingly to counter her. For example, if she goes for a "machine gun" type build where she builds a lot of attack speed and on-hit items with the use of Pix, Faerie Companion, consider building Randuin's Omen and Frozen Heart to counter her.
On the other, pay attention to which lane Lulu can go. While she's normally played as a support, she can also be played as an ADC due to her Pix, Faerie Companion and disabling kit. She can easily go top and middle lane due to her crowd control and poking kit in addition to Wild Growth and Whimsy's speed boost makes it incredibly difficult for melee bruisers and assassins to kill her quickly. Consider playing sustain based champions like Nidalee, Yorick, and Vladimir instead to counteract her crowd control and poke.