Brunt's driving edge gave Bryce a simple return get and when Teresa Graves bowled Sterre Kallis and Ami Campbell was gotten at mid-on in the equivalent over, Diamonds were six down and battling.
Left-arm spinner Kirstie Gordon, who took four wickets against champions the Southern Vipers on Saturday, Rachael Heyhoe Flint Trophy 2021 went wicketless this time and it took the returning Bryce to represent Gunn through a top-edged draw to midwicket.
With the association broken, Diamonds lost their last four wickets for 20. Yvonne Graves had Beth Langston got at mid-off, individual offspinner Lucy Higham bowled MacDonald for 39 and followed up by having Katie Levick gotten behind.
Lightning were shaken by a splendid opening burst by Brunt, who thumped ease opener Sarah Bryce's off-stump briefly ball duck - the second in as numerous counterparts for Lightning's top scorer in 2020 - and had Kathryn edging behind.