Twenty-year-old Bethell smashed an unbeaten 56 from just 16 balls, including a 15-ball half-century, the quickest for the Bears in the Blast, to lift his side to 219 for four.
Ed Barnard (48, 29), Dan Mousley (43, 26) and Alex Davies (36, 29) had already laid the foundation for a solid total but Bethell turned solid into spectacular with a furious onslaught of seven sixes, including four from Ben Sanderson in the last over.
Steelbacks then meandered to 129 for nine (Bethell adding two for 18 to his batting heroics) to suffer their eighth successive T20 defeat to the Bears. The only negative of the night for the home side was an injury to fast bowler Richard Gleeson who limped off the field, apparently with a hamstring injury, after delivering his seventh ball.