Marcello Trotta started the blitz with a Luis Suarez-style header from distance just before the break.
The Bees extended their lead a minute after the restart when Adam Forshaw jinked inside his man to pass the ball just inside the upright.
Chelsea loanee George Saville made it three just before the hour when he killed Alan McCormack's diagonal ball to curl a beauty into the far corner.
Donaldson rubbed salt in the wounds of the woeful Alex defence minutes later, beating the offside trap to tap home from close range.
And Brentford made it two five-goal hauls in a week after 72 minutes as Donaldson raced onto a slick six-man move to fire beneath keeper Alan Martin.
Bradden Inman had the visitors' only real glimpse of goal but sliced wide when set up by Byron Moore.
Substitute Vadaine Oliver also fired wide from point-blank range as the Bees took almost total control to put five past Crewe for the second time in as many seasons.
- Sports & Recreation
- Adam Forshaw
- Alan McCormack