To rekt with Nova, you need to start playing clever. Snipers are not fighting in the front lines. They go around and hit where it hurts most.

So here are some tips for ya.
1. Most important, DON'T FOLLOW YOUR TEAM ALL THE TIME. You're a loone wolf, care only about yourself. Becouse there is no use of you, when you're dead.
2. You don't have to kill minions, just stay close to the place where there are fighting, it will give exp too.
3. Nova never goes first to team fights, nor she goes last. She joins just when she wants to.
4. Use your terrain, places where you can hide were made for a reason.
5. Be fast, or be dead.
6. MINIMAP, LOOK ON IT.

So you know the basics. Now about how to kill stuff.

Hunter must pick his targets carefully, or they will run away.
Most easy are the ones who walk alone, and most hard ones are with lots of hp.
Also when your prey is running, don't chase it instantly. Wait for cloack to work again. Becouse you never know whats behind the corner.
When engaging on lower lvls, it doesn't matter if you start with Snipe or Strike, but after 16lvl you always start with Crippling Strike.
Sometimes your team will focus on a tanks instead of more important targets. In this case there are two options:
Under 16 lvl, go for ADC or Support, above 16lvl use Cripling Shoot on the tank. So your team will kill him faster and be able to focus on other targets.
But when you see, that your team is going to loose this fight, RUN. Remember you're useless when u die.
Also remember to use precision strike, on the start or in the middle of team fights. Enemy will want to avoid the place of impact, and move away instead of attacking your team. That helps A LOT.

Sometimes hunter might become the haunted, and while this happens ADV. Cloacking will come in handy.
It not only makes you run faster, but also can fully heal you in 30-40 secs. While in use with terrain and places to hide, you can even evade a whole enemy team.
After that you can go back to your base, regen mana, and start poaching again.
Also try to avoid places with only 1 way in or out.

With a little practice, this build will get you "Hero of the storm" rank quite often. Ofc your team might start talking some ****, like "Nova your useless noob". But still don't bother with them, just kill lots of enemy heros and you should win.

Good luck, and good night.