Levi hit 69 off just 34 deliveries, while Waller's four wickets cost just 16 runs as last season's beaten finalists started this season's competition with a win.
Somerset, put into bat in front of a crowd of more than 5,000 were given a good start by Craig Kieswetter, who hit a swift 20 - including two sixes - before he was caught at short fine leg by Chris Wright off Chris Woakes.
Chasing a victory target of 192, Laurie Evans and Varun Chopra had taken the total onto 40 in the sixth over when Alfonso Thomas bowled Evans for 17.
Chopra went down the wicket to George Dockrell and was smartly stumped by Kieswetter for 37, before Waller picked up his third wicket when he bowled Rikki Clarke for six with the total on 72. Fittingly, Levi ended the Bears innings on 128 when he caught Wright at long-on.
Elsewhere, Surrey opened their Friends Life t20 campaign with a 17-run victory over Essex at The Oval.
The hosts managed just 128 for eight but defended that total, despite the visitors starting their chase with a 44-run opening partnership between Mark Pettini and Ravi Bopara in just six overs.
However, when Pettini edged Jade Dernbach behind the wicket, the complexion of the game altered and by the time the score had been doubled, Essex were in deep trouble. The visitors lost seven wickets in 10 overs, three of them to the spin of Gareth Batty, who finished with three wickets for 20 runs.
As the panic button was pressed, a horrendous mix-up between Graham Napier and James Franklin found both batsmen at the same end, with Napier departing for one as the innings disintegrated.
Surrey's significant partnership came from their sixth wicket pairing of Gary Wilson and Zafar Ansari, who added 40 after coming together with the score on 68 for five shortly after the halfway stage of the innings. The pick of the Essex attack was Napier, who found line and length from the start to pose problems for batsmen throughout his four overs.