Arsenal is now six-game unbeaten following their 3-1 win over Southampton in a tough Premier League fixture played outside London.
Armstrong got a third-minute strike from a well-worked set-piece assisted by James Ward-Prowse to make it 1-0.
Pepe equalized in the eighth minute to make it 1-1 after getting assisted by the captain of the night Hector Bellerin.
Then Lacazette passed the ball to Bukayo Saka who makes it 2-1 in the 39 minute of the game
In the 72nd minute Saka provided an assist for lacazette to seal the game 3-1
Arsenal have now temporarily moved to eighth while Southampton dropped to 11th on the Premier League table.