Playing their first home match of the season, the Pilgrims found themselves 1-0 down at half-time, before five further goals after the break.
United's goals were scored by Rueben Fogg (from the penalty spot) and Lake Taylor-Peak.
Boston: Sam Buckland, Archie Bradford (Jack Dixon 67), Tyreece Keating, Dexter Tomlinson (Finley McDaid 75), Elod Nadasdi, Finley Ging, Toby Foreman, Rueben Fogg, Sam Finch, Sean Bray-Smith (Lake Taylor-Peak 75), Joe Todd (Oscar Broom 85). Sub (not used): Alfie Harney (gk).
Referee: Istvan Mod.