especially with the new update, it seems completely impossible. I've tried the macao strategy, but now portugal wants me to cede the entire fucking state of guangdong! yeah i banned opium but now i have to get that whole fucking state back! is there another way?
Most of the times I've won its by relocating the capital to like ningxia or somewhere west on day 1 then reorganizing the armies so the green armies that are all infantry divisions are also packed full of conscript levies and use them exclusively on the defense with the proper banner armies receiving proper artillery and upgrading the calvary to like dragoons or lancers and using them on the defense in the main front or for being sent to defend against landing attempts in vassal states.
Also rush tech for line infantry while spending the time in the lead-up to and during the actual war building either artillery factories and/or firearm factories and resources to feed into them.
Focusing on absolute defense to bring the war to a white peace draw wins the war in your favor ultimately. Even taking an income hit by accepting to pay war indemnities to push the negotiations over the edge to end the war is acceptable as it doesn't fulfill any of the loss qualifications for the opium wars quest.
I've also occasionally won by cheesing the brits by counter-invading random fucking places to draw their armies out and even one time invaded Ireland and almost capitulated them before they kicked me off and those bought me enough time and war exhaustion points to get a ceasefire without moving the capital