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Please note this is a guide for those wishing to try off meta builds. I do not advise playing this in ranked unless you have tried and tested it. These builds have been tried and tested by myself and/or my group (NOOBN).

ADC Janna does a lot of damage mixed with the extra damage to movement impaired champions and bonus MS working with her passive. She is good mainly early/late, but can be played well into late game.

I like to play with a healing support as she is quite shuishy. My personal favourite is a good Soraka.

In team fights you are low range so your main goal is to use your Q to help your team before doing damage. She can be played either utility ADC style or assassin if you go full damage.

A lot of the time they wont expect your damage and you can get a few free kills by abusing that.

Pros:

High damage
High movement speed
Can escape easily if caught out
A lot of CC

Cons:

Low range
Late game falls off a bit
Extremely squishy

Early game you have to farm and try to hit your q/w. This will give you bonus damage and also surprise the enemy. Try to fight in ally minion wave to help your damage.

The best supports to play with ADC Janna are healing supports like Soraka or Nami. You are very squishy and the extra health helps you to survive early when you will get one shot.

Mid game you will want to get the turret as soon as possible and then rotate when possible. Top lane is a good choice for Janna to rotate to as they are often less damage and you are less likely to get caught out.

Late game you will want to play like a utility ADC. Using your CC and your E to boost your damage across the enemy team. Your low range and immobility means you need to be careful not to go too deep in the fights, but games can be won later with Janna ADC.