They switched off in the second half and allowed the visitors to reduce the deficit in the closing minutes, but manager Kenny Jackett will be very pleased with a third successive victory.
The Swans went on the attack straight form the kick off and Boston skipper Paul Ellender brought down Forbes as the former Luton winger raced through the middle.
Austin McCann argued too forcibly with Referee Jarmal Fingh and he was shown a yellow card before the official moved the ball ten yards to the edge of the area.
Up stepped Trundle to lash in his 11th goal of the season and give his side a dream start.
The visitors never learned from that error and when Trundle fired in another free-kick it had Nathan Abbey stretching to tip over his own crossbar for a corner.
A blunder by Abbey in the 20th minute gifted the Swans a second goal.
A long range effort by Alan Tate was pushed away by Abbey when he had time to gather. The loose ball went out on to the flank to Forbes and as he shaped up to return it to the danger area, Abbey slipped and was lying flat on his back as the ball sailed into the empty net.
Seven minutes later a superbly weighted pass by Trundle sent Goodfellow free and he finished with a crisp shot that flew underneath the diving Abbey.
The Boston keeper was the busiest man on the field and as the interval approached he was forced to palm away a rasping drive from Paul Connor.
Early in the second half Trundle evaded a challenge and flicked the ball goalwards past the onrushing Abbey, but it struck the foot of the post and went behind.
Boston substitute Jason Lee struck an 86th minute consolation goal for the visitors after the ball had come off the woodwork.
And it took a superb leap by Swans keeper Willy Gueret from a Paul Ellender cross-shot in the last minute to prevent the Pilgrims reducing the deficit even further.