This is my way of playing nidalee and I have heard loads of people telling me how wrong it is to play her with so much AP, And how much I suck cause I havnt bought a Guinsoo. But hwoever I still belive this is the best way to build her, and it works for me so why dont you try it out ? ;) have so far won 9 out of 10 ranked matches with her :P
You are all welcome to comment, and I excuse for any spelling mistakes, English is not my native language... (and plz try this out before voting!)

The early mana trouble that you have with nidalee gets smaller when you start of with a tear, I choose not to build it into and archagel untill later because it is rather expencive and you dont get a whole lot of AP.

(sometimes I through in a Mejai's if I know that its going to be a good game, but think twice before buying it, it can be a waste...)

The early deathcap is what makes youre day, when you get this youre enemies will seriously start fearing youre spear, and by this time youre friends will also love youre heal.

The Lich Bane gives you some additional AP, but the UNIQUE part of it works perfectly if you get into a 1v1, then you just jump on them with youre cougar and watch their health drop.

The rest is pretty self explaining, more AP -> MR shredder -> more AP.

The way I use the two forms are very simple, I run around i cougar form cause its faster and therefore safer if u run into someone. Whenever an enemy is around I throw spears, heal friends and just tries to get damage onto their squishies. This can give u an advantage in team fights because, youre enemy has lost loads of health even before you get into the fight.
When a team fight starts I try to do like a healer, stay behind and heal, when someone gets low (try to heal youre ad carry, it also gives attackspeed), then I ofc use my spear, and try to throw it on to their squishies. I never use cougar form in a team fight cause, the enemy team can turn you into dirt in seconds.

This sequence is what i primarilly do, but sometimes I choose to max out the heal a bit earlier, sometimes I only grap 1 point in heal and then all out in the spear. It depends a whole lot of youre positioning and who you are up against...
However it is always smart to grap youre ulti whenever posible, it makes it a whole lot easier to farm minions and creeps in the jungle.

I normally stay in my lane a loooong time, I try to get a teammate to help me blue so I can spam my abilities and keep myself high on HP. When you reach level 9 you can take down the blue youre self, just heal go into cougar form and spam youre abilities. When the AS-buff from the heal goes off you, go back in human and use all youre abilities again...
When you have taken it you heal yourself up and go to war.

I wont say a whole lot more except that you should try this out, im sure you will like it if you have the spear skill it takes... :P