This is how I build/play Nidalee at lev 1 get boots and 3 Mp pots then you are good tell lev 6 then you get Sheen to make your cougar hit hard then a Vampiric Scepter to help you focus on healing team other than you so much and at this point you can solo buff mobs np golem buff helps.As soon as you can get a Mejai's Soulstealer because you will be killing players and why not get stronger wile doing so.
Items you build into after this point really depends on how other team is built but I always end up with a Boots of Mobility, Trinity Force, Mejai's Soulstealer, Malady and last but really not least Guinsoo's Rageblade that item helps this class a LOT!
Remember Primal Surge is your main skill to build into first but you still want Javelin not more than 2 points behind Primal Surge Takedown is your 2nd main skill.As far as mastery gos you should know what to build into with offensive but in Defensive put into magic resist then work for doge+ and Nimbleness you got really good doge in cougar so put more into doge than Def.
Rally and Exhaust are the only 2 spells for this summoner IMO doping rally wile in Cougar can =Win I put a mastery into rally to make my cougar do O,O damige with her skill it makes a noticeable difference and Exhaust is good for any melee class really.
Runes I did Mark:Armor Penetration Seal:Doge+ Glyph:Atk speed+
Quintessence of Alacrity x2 and 1 Quintessence of Evasion you can do Glyph AP+ or - cooldown both of them work just as good but I like the little atk speed+ I get with my Glyph.
Some players think doing all AP with her works and its not bad but its not the best for her she is magic/melee and I found having a Mejai's Soulstealer is MORE than good for a AP boost with her.
Any suggestions/questions please post a reply and I will see what I can do to help.