Kill 100 Monsters
Easy, just play the game and you'll get this one fast enough.

Unlock the first sniper
Go to the menu, Press armory and buy your first sniper.
The one I chose was the Lvl 6 | Mag 14 | Dmg ultimate / default one.

Destroy five cars
Shoot the cars in LvL 1. I did this after i unlocked the sniper.
I just went back and shot 5.
It can be done in multiple runs.

Finish the game
^kinda says all

Unlock the final gun in the game.

Buy the ''From The Future'' assault rifle. It's $5000.