I have played Black 2 extensively, and I can say your Magby will be absolutely fine if it's a bit underlevelled. As a matter of fact, I myself recently used an underlevelled Magby for Burgh -- in a Nuzlocke, no less. Most Pokemon in the Gym are Sewaddle/Swadloon, which have a 4x weakness to Magby's STAB. The Combees shouldn't pose too much difficulty either. The only problems I can see you having are with Dwebbles and maybe Burgh's Leavanny, but that's what teammates are for. As long as your Magby is upwards of Level 18, you'll be fine.
If you need a place to grind, the Breeder on Route 4 gives quite a bit of experience every time you beat her -- though her two Pokemon are a Drilbur and a Marill. Alternatively, head to the outdoor region of Castelia Sewers and grind Audinos.
Hope I helped!