The first half wasn't as easy as you said, because Barcelona struggled to score just one goal on the 40th minutes of the match by Lewandowski with a clinical finish to put Barcelona on the lead after he got an assist from Kounde. Real Betis also had some good chances but they couldn't convert it to a goal, and the first half ended 1-0. It's possible for Barcelona to score more goals in the second half.
Real Betis came down with a defensive strategy, which then made it difficult for Barcelona to find a gap. Luckily before the first half ended they were able to score a goal to make them more comfortable playing in the second half.
However, they also have to be vigilant, because a 1 goal advantage can change in a very short time. Although they currently dominate the game, they must be vigilant with counterattacks that Real Betis can do at any time.
Apart from the goals scored against them, overall Real Betis can implement a pretty good defense in my opinion. They just have to find a gap or luck to be able to counterattack effectively.