The Bees were 3-0 up at the break courtesy of two headed goals from Alexander, who steered home two pinpoint crosses from Jake Bidwell.
In between Sam Saunders got on the end of a goalmouth melee to coolly slot home from close range. The visitors rallied after the break and Dave Winfield’s powerful header from Grant Basey’s free-kick gave them hope.
Diagouraga restored the three-goal cushion midway through the second half when he latched on to Marcus Bean’s low right-wing cross.
But Wycombe responded minutes later when Joel Grant converted a corner at the far post. But the pick of the goals came in the dying minutes, Alexander clinching the match ball when he got on the end of Clayton Donaldson’s deep left-wing cross to smash home an unstoppable volley at the far post.
It might have been worse for the Chairboys had Jonathan Douglas’s curling drive not hit the post and Myles Weston’s 30-yard shot been an inch lower.