A lot of British success in the OTL Opium War was predicated on the help of native Chinese who, for various reasons (pre-nationalism period, disillusionment with the government, converts, pirates etc.), offered logistical/scouting help to the British. Certainly the British military couldn't have overrun cities such as Canton without at least some assistance from local hongs. I doubt this would change much even with Qianlong.
I think the single most important thing the Qianlong Emperor could have brought to the fight (as opposed to OTL Daoguang) was probably much better control between Beijing and the armies/navies in the field. OTL poor communication between Chinese provincial defences, as well as deliberate obfuscation between field generals afraid of punishment and Beijing, greatly hampered Chinese defensive efforts. Strong leadership from Qianlong would have better coordinated these efforts.
In addition, Qianlong was one of the few people in China who understood the extent of the British Empire, esp. in India; it's possible that he could have arranged for some coordinating attacks from the Sikhs or the Gurkhas to distract the British.
Military-wise Qing China and Britain would obviously be at a more level playing field, though the Manchu core of the Qing military was already in decline by the late Qianlong era. Qing cannon, guns and fortresses would obviously be less inferior compared to OTL, but basic Chinese weaknesses such as poor discipline, lack of reconnaissance/initiative and tendency to rout in the face of a bayonet charge would have remained. Most importantly, British naval superiority would mean that the British got to pick and choose their battles, while the Chinese had to defend everything.
I think British victory over the Qing during this period is distinctly possible, but would require quite a bit more resources than OTL - or conversely, less ambitious war goals.