How do players get so many duels in such a short time??
I have seen as much as 100 in first week
Not that difficult really. Active players visit the game at least a few times a day. In the morning they do six duels, six kills. If they have enough friends they can level up by visiting all their friends once, receiving six more duels (six additional kills). Since one gets a bullet per hour, in the afternoon they can play six more duels. And in the evening another six. That's 24 duels, and if you always play against easy opponents you get 23-24 kills from that. That's well past 150 duel kills a week. And now that you can receive refill duel ammunition packs from friends, you can go far beyond that. Even if you don't have enough friend to level up daily and get extra bullets from that, active duelling should get you past 100 in a week.
I have 55 friends and have managed to get to around 180 duel kills a week (without using the Refill Duel Ammunition).