With as much AP as this build produces and the survivability of Janna, she should both be able to get the edge on kills and keep it as her AP ramps up with Mejai's Soulstealer and Archangel's Staff. As time goes on she'll be harder to kill with Lich Bane and Zhonya's Hourglass.

If this build carries to endgame then you will want good crowd control on your team. Other than the hourglass and Janna's personal survival capabilities, this build will be easy to oneshot. To do damage, shield yourself and then launch Whirlwind, Zephyr, Ignite combo and then run away. The shield should cover between 900-1000 damage at endgame, so it will likely be your best survival tool. If it's more than a 1v1 fight you may as well pop rally at the beginning of your combo.

Also, by the end of the game Lich Bane will be doing an obnoxious amount of damage if it is set off. Therefore, try to get one basic hit in after each combo if you feel safe enough.

If the odds are largely against you, but you think you can get a kill with a couple more hits, pop hourglass to absorb their bit hits. When it falls of, shield, kill your target, monsoon, and then run like mad. The hourglass should absorb most slows/stuns and whatever big damage cooldowns. Therefore, your shield should hold long enough to get off another combo of sorts and then hopefully monsoon to get distance.

Most builds for Janna run her largely as support. I can see this for some party makeups I guess, but I prefer doing a log of damage with Janna and being in the thick of it.

With all that said, this build does fall short on the survival side sometimes. Occasionally I'll be a bit too slow to shield and get crushed by Annie. If this happens too much then feel free to sell Mejai's Soulstealer for Warmog's Armor. Basically, if you arrive at endgame and Mejai's Soulstealer drops below 10 points then you should probably sell it for Warmog's Armor starting with Giant's Belt. That should eventually put you at about 3100 HP and with a shield around 4000. It's less likely you'll be able to solo enemy mages, but you'll be able to move around more confidently.

Another great thing to note about Janna is that her Whirlwind is great to keep Banshee's Veil at bay on an AoE scale.