Centre-back Harlee Dean completed the rout as the Railwaymen hit the buffers, only managing an early second-half consolation through Mathias Pogba.
Donaldson broke the deadlock after 13 minutes, reacting quickest in the box to lash an unstoppable angled drive past goalkeeper Alan Martin before Moroccan striker El Alagui, a close-season signing from Falkirk, opened his Bees account with the second, capitalising on a defensive mix up to rifle home.
The Londoners made it three before the break when El Alagui was in the right place to convert Shaleum Logan's drilled right-wing cross.
The visitors responded after the break, Pogba firing home Byron Moore's right-wing cross from close range as the rain continued to pound Griffin Park.
Dean powered home a header with minutes remaining and Donaldson grabbed his second when he cut in from the right and saw his low drive deflect past the unfortunate Martin.