This is a quick guide to how I play Lulu mid. Because Lulu is really new, I haven't had possibility to test what items are the best and if my runes and masteries are the most efficient ones, but I've had success with this build.
Lulu mid has in my opinion high skillcap and high reward. Her strengths are both good soloing and teamfight capabilities. She is at the moment (in my opinion) viable for midlane, but she will probably get nerfed sooner or later.
This is my standard mage runepage. Gives nice manareg and AP for being aggressive at midlane.
Again, this is my standard mage mastery page. Not much to say.
Flash and Ignite for offensive playstyle. Again basic stuff for all mages.
I pick equally many Q(Glitterlance) and E(Help,Pix!) for having safe damage output and utility from the slow and shield. I pick 1 in W(Whimsy) for escaping ganks and catching fleeing low health opponents early. Ofcourse max ultimate asap.
I'm not exactly sure which items are the strongest for Lulu, but I've found that I always seem to be going for something like the build above.
What I want from my items:
1. AP for damage
2. Cooldown reduction for better ability spam
3. Survivability to not explode in focus
4. Manaregen to not run out of mana after using abilities
5. Useful "actives" or procs to increase solo damageoutput
I haven't tested building Hextech Gunblade instead of Will of the Ancients but I assume its good :)
But go ahead and build what seems to work best for You!
Basics of how to play
Mid Lulu's playstyle is somewhat similar to Mid Karma, as their offense and defensive capabilities are somewhat close to each others. So if you know how to play Karma mid then Lulu will most probably not be a big problem.
Timing is the key.
You will need to know when to use which ability, to be able to maximize the damage. You will need to know when its best to use Whimsy(W) as speed boost to escape chasing enemies or as a polymorph to prevent threatening damage for 1.5 seconds. You will need to know when its time to deal damage with Help, Pix!(E) or when you need it for shielding damage.
Farming and harassing with Lulu is very cost effective, since her abilities cost so little mana. My runes and masteries allow me to basically spam abilities for harassing and farming early game.
Fighting with Lulu is a bit complicated, but when mastered, she is almost impossible to kill.
Lulu focuses on dealing consistent damage instead of having a huge nuke. You will need to harass quite much (which isn't a problem) to be able to control the lane.
Harassing: To deal damage early game use E(Help, Pix!) to tag the target then use Q(Glitterlance) for free damage and slow, then try to get some autoattacks off if possible (remember to move out of minion aggro fast). This can all be done at max range to minimize incoming counter harassment.
If you want to get a kill it will most likely require some baiting with ultimate or Whimsy(W). This will require getting closer to the target than harassing to keep in range after possible escape attempts. Use ultimate only when in desperate need of CC (to get Q or E off cooldown in time for kill) or in desperate need of health.
Her Q and E cooldowns are fairly short so the consistent damage output with high AP will be good. Also her CC and supporting potential is extremely useful in teamfights.
Teamfights and Supporting
In teamfights your main focus is to disrupt/block strong ultimates with polymorph and keeping focused teammates alive with shield and ultimate, while dealing damage with Q(Glitterlance). This is much easier said than done, but Lulu's supporting capabilities in teamfights can easily carry the game.
Do note that you shouldn't save your abilities for yourself only! Since depending on how fed/farmed you are, you will most likely NOT be the main damage output on your team. BUT do not hesitate to do whatever you can to keep yourself alive, if caught in a bad position.
1. Spam Q(Glitterlance)
2. W(Whimsy) to debilitate high damage dealer, block strong ultimate or to speed up chasing or escaping
3. E(Help, Pix!) to shield heavily focused teammate or to finish low health enemy
4. Ultimate to CC clutched areas and heal focused ally
Thank You for reading!
I hope you found this guide useful and enjoy pwning mid ;)
Also I'm prepared to answer questions in the comments and explain further my thought process if asked for.