Head to the highway to grab the 287 bus at either 6am or 1:45pm. The 287 heads directly to Sonsonate twice per day. These times are when it leaves La Libertad and so it will pass the highway in front of El Tunco ~15-30mins after. Be aware that sometimes buses leave early in El Salvador and therefore if you are planning on grabbing the 6am bus it could arrive in front of La Libertad not too much past that. If you are headed to Santa Ana, you can change to bus 216 in Sonsonate which will take you to the Terminal de Buses Francisco Lara Pineda when you arrive in Santa Ana.
If you are heading to Juayúa or Ataco you can change buses in Sonsonate for the 249 which goes to Ahuachapán. This bus will pass Juayúa first and then Ataco along the way – aprox 20-30mins after you leave Sonsonate.