For the Premier League leaders, the result also constituted a 10th straight win in all competitions and an eighth consecutive clean sheet.
They have now scored 11 goals and conceded none in the Champions League so far as they maintained their perfect start.
“The most pleasing thing probably is not the record, it’s the mindset of the players, they are talking there about how we can still do better,” said Arsenal manager Mikel Arteta.
The match also saw a moment of history as Arsenal’s Max Dowman became the youngest-ever player in a Champions League game aged just 15 years and 308 days when he came onto the pitch in the 73rd minute.
Stand-in captain Bukayo Saka opened the scoring from the penalty spot on 32 minutes after Slavia captain Lukas Provod had handled the ball in the box clearing a corner.
Merino made it 2-0 from a quick breakaway 30 seconds into the second half as he volleyed home at the near post after Leandro Trossard’s superb pass found him unmarked in the Slavia box.
The Spaniard then beat Slavia goalkeeper Jakub Markovic with a header from Declan Rice’s cross on 68 minutes.
“I was really happy with the win, four in a row without conceding a goal,” said Arteta.
“Very short sequences of play, a lot of fouls, a lot of restarts, very difficult to get any fluency in the match but if we were patient and emotionally controlled, we were going to have our chances,” he added.
The loss leaves Slavia, the Czech top-flight leaders, with two points from four games in the Champions League. (AFP)
