I use three methods, depending on times of travel etc. There's a 16:30 bus from La Linea to Torremolinos that takes about 2.5 hours, its about 15 Euros for the bus and 2 Euros for the Metro.
Last week I used a car share app. called Bla Bla Car to get a lift to a Torremolinos hotel before a morning flight, that was £7.
Or if I need to there's a driver I use who charges £110 from La Linea to the airport. He takes the fast route and the toll charges are included in the price.