It'll be best that they play a defensive game but it's certain that Madrid would be a better side but there's no harm in giving it a shot.It's football, and anything can happen so there's a possibility that they can still turn the game to their favor even though the odds are pretty low.
if Betis plays too defensively with solid defense, it will only give Madrid more opportunities to win the match.
The last meeting between the two ended in a goalless draw. Betis knows how to contain Madrid. it's just that they have to hope that Madrid is not in top form.
if they let Madrid control the ball, Betis also has to plan counter-attack options with the gaps in Madrid's defense that focus on attacking. and that requires players with speed to match Madrid's defensive transitions.
it may be difficult, but Betis has done it. so there is nothing impossible to do.