The goalfest started after just five minutes when José María Movilla found the back of the net, with Michu the only player to score twice as Osasuna were humiliated.
Yet saving a point was a relief for Espanyol as they had to work hard to cancel out Carlos Vela's two goals inside the first quarter of an hour.
An own goal by Vadim Demidov just before the break and a strike from Vladimír Weiss two minutes after the restart saw them earn a draw.
Roque Santa Cruz and Ruben Castro struck within three minutes of each other in the first half before Benat converted following a fine move after 59 minutes. Marco Ruben claimed a consolation when scoring a penalty deep into injury time.
Betis are now 10 points clear of the relegation zone with seven games remaining, while Villarreal - who lost Angel Lopez to a second yellow card - remain just four points above 18th-placed Real Zaragoza.
Miku and Diego Castro were the goalscorers.