The Pilgrims lost Shiels to a second yellow card on the hour mark - but produced a dogged rearguard action to keep newly-promoted Banbury United at bay.
Tom Platt was denied by Jack Harding in the first half, while Sam Long excelled to keep out an Alex Babos header in the second period.
Kick-off was delayed until 8.00pm, following torrential rain in Oxfordshire, but referee Ruebyn Ricardo allowed the match to proceed and he was rewarded with two teams going all-out for three points.
Jordon Crawford's low drive flew wide in the second minute, while Harding was soon called into action to repel Platt, following Jake Wright's cutback from the left.
United's evening looked set to unravel when Shiels was handed his marching orders on the hour mark - receiving a second yellow card.
Long made a brilliant save to thwart Babos in the 67th minute - but clear-cut chances otherwise remained at a premium - and the Pilgrims defended resolutely to keep the Puritans at bay.
Banbury (3-4-1-2): Jack Harding; Ben Radcliffe, Jay Williams, Connor Roberts (Sam Brown 79); Jak Hickman, Josh Smile, Giorgio Rasulo, Ben Acquaye; Alex Babos; Jack Stevens (Henry Landers 65), Morgan Roberts. Subs (not used): Ben Taylor, Aidan Hawtin, William Owens.
Boston (3-4-1-2): Sam Long; Ben Pollock, Luke Shiels, Jean Leroy Belehouan; Ellis Myles (Keenan Ferguson 79), Tom Platt, Will Atkinson, Alex Brown; Brad Abbott; Jake Wright (Jaanai Gordon 86), Jordon Crawford (Joe Leesley 83). Subs (not used): Lebrun Mbeka, Micah Obiero.
Referee: Ruebyn Ricardo.
Attendance: 812.