Dayle Southwell grabbed a first half hat-trick as the Pilgrims took a five-goal interval lead, with flying wingers Jay Rollins and Kaine Felix also on target.
Former United youth teamer Aaron Eyett cut the deficit after the break, before Scott Garner scored the Pilgrims' sixth to cap a dominant display for the visitors.
Boston United (4-4-2): Matt Urwin (Alex Brown 46); Cameron Dear, Scott Garner, Zak Mills, Liam Marrs; Jay Rollins (Dymon Labonne 46), Kyle Dixon, Grant Roberts, Kaine Felix (Jamie McGhee 46); Dayle Southwell (Louis Saebe 46), Jake Sheridan (Harry Limb 46).
Referee: Alistair Wilson.