Be patient play defensively and just last hit creeps like you're supposed to do... you you have an opportunity to hit them take a shot or 2 and then withdraw.

Nunu's early game is powerful, but not an issue if you play defensively. Teemo, blind is gay. Specifically what kind of problems are you having against them? Here's some hints to help:

Ghost/Cleanse and Ghost/Flash are your two biggest friends. I prefer ghost cleanse since there is so much **** out there that will ream you in a stun/snare/root. Additionally pick up items like Madred's bloodrazor, Sheen > triforce, Mercury Treads, Malady, Phantom Dancer, and Banshee's. Also try building your skills in the order you'll most likely need them. If you're getting hit hard, put more levels in your heal early.

E, W, Q, E, Q, R. That's a basic build that works out nicely. If you do find yourself being hit hard you can swap out your first Q rank for an extra heal rank.

Lastly, a good starting item is Doran's Ring. The health is quite nice, but it;s the 5mp5 that really saves the day.

Edit: Read your 2nd post. Late game Nidalee falls off from her powerhouse mid game. Unless you're going AD, expect to sit back and throw spears/heals and only engage in cat when you are going to get a kill. AP Nida spears can half hp other carries if thrown right, so shes still pretty powerful. AD Nida's auto attack can be pretty vicious. As far as chasing toons down, Teemo's sprint ability is until hes hit, if you can land a hit on him then you should have him.

Late game you really shouldn't be chasing/fleeing by yourself tho, unless you just destroyed their team and you're chasing them for the ace, or otherway around.

^^ Yeh, new jax HURTS!

Seen one yesterday 1lvl lower than me, 3/4 his hp, I was full. Speared, healed, cat, did my combo, he was half, I was 1/3. o.O

Glass Canon Jax builds are very tanky atm, which kinda defeats the purpose of balancing large AD items from large HP items.

Nidalee will lose to most melee carry if fought fair and square.

Toe to toe combat on nidalee only works during mid game.

(lv 6-12 sometimes 14) after that you will start to see how everybody start owning you one on one unless you are way fed)

If you want to kill someone, best way is to hit them with javalin until they are at half health.

After they are at half health, now you can jump in as cougar.