Not Updated For Current Season
So, welcome to my first guide. Nidalee was my second purchase, and like the first,I have yet to regret it. She is an active hero - being able to plant traps, heal, throw spears for heavy damage, and then shapeshift for a whole new set of abilities.
This is not a direct 'how you must play' Nidalee. This is how I've enjoyed some success with her in both 3's and 5's.
I play pretty conservative until level 6, when I get Aspect of Cougar. From there, use pounce and swipe to ravage minions and harass enemy heroes - she can farm up gold pretty quick.
You want enough mana regen to constantly cast your spears early game - the better to harass your foes. The Magic Pen just makes them hurt all the more early and late game. Ability power is just adding fuel to the fire - and making your heals stronger.
I play with mostly Utility Masteries, taking the magic penetration in offensive, but focusing on CD reduction and mana regen. She's a hit and run opportunist, I get squashed trying to jump right into the middle of things. (especially if taunted)
Another possible alternative would be 21 / 0 / 9 - taking the last of Assault for more spear / cougar damage.
I've been taking Doran's Ring instead of a Meki Pendant lately, as I've been taking the middle lane. Yes - Nidalee can take middle lane. Just play conservative, and harass with your spear, until level 6.
The last item I usually change out sometimes. Rod of Ages if I'm having mana issues. Ryali's Crystal Scepter if they keep getting away. (all of her abilities but 1 in feline form do magic damage).
You could also work out a Lich Bane - because with the 500 someodd ability power, taking down towers gets easy.
Ghost - for both chasing and running.
Exhaust - when they try and run or you need to make a getaway from a single person. With the Mastery, it makes everything you do hit harder.
I play Nidalee as a hit and run opportunist hero. Strike from the bushes, or into bushes if they trigger a trap. Your traps reduce their magic defense - don't be afraid to run away, she doesn't have the health to stand up to the burst of most heroes, and assuredly not prolonged exposure to fast damage. In longer games, concentrate on throwing your spears, and tossing heals - with a full build, your heals will reach up into the 900's.
My highest damage spear, at this writing, was 1236 against a Caitlyn who fired her net to go backwards, and got caught at the very tip of the throw.
Nidalee is versatile. Nidalee is sexy. Enjoy the game!