To do this, the Qing would merely have to push a few British invasion forces back into the sea.
'Merely' pushing a few British invasion forces back into the sea is almost ASB already with a post-1800s Qing PoD. I'll repost something I wrote in the Paradox forums...
1) The Chinese were completely outclassed militarily. British guns had farther range than Chinese coastal forts (many armed with 17thC cannons) and Chinese junks. The UK also had a massive advantage in mobility, which meant they could bypass Chinese defences where they were strong (like at Dagu, Canton) and hit them where they were weak (like at Fujian and Zhejiang). Britain's soldiers were veterans who had access to the best weaponry of the era while Chinese soldiers were largely conscripts and convicts. Malaria was responsible for the vast majority of British casualties during the war.
2) The British received plenty of help from locals. The Chinese ban on non-Canton trade created a large smuggling market between Westerners and locals, the latter often very willing to help out when the British needed help. Missionaries like Karl Gutzlaff were perfectly in command of local dialects and could thus exploit native knowledge even on what one would consider 'foreign soil'.
3) Chinese leadership. It was atrocious, though you do have to cut them some slack for dealing with something they had clearly not experienced before. They frequently conceded important ground (such as the hills outside Canton or Dinghai on which artillery could be placed) with little struggle. Entire wings of an army could simply disintegrate in a moment or even worse, retreat without fighting or even without orders. They were also prone (and this is a problem that persists all the way to the Boxer Rebellion and beyond) to abandoning prepared positions when faced with - or even because they expected - a bayonet charge. Sure, many killed themselves or died fighting but that certainly doesn't absolve them from basic errors of judgment.
On a strategic level, Chinese generals regularly misled Beijing and each other by concealing losses and exaggerating victories. The British decision not to approach Canton in June 1840, which even today is sometimes written like a Chinese victory, was certainly interpreted as such by Lin Zexu and Guan Tianpei, which meant that commanders in Amoy and Dinghai were completely unprepared for the British assault coming their way.
4) Qing strategy. Daoguang was not the most decisive of people (some say because he was an opium addict himself) and his attitude towards the British swung from begging for peace to war, often due to the misleading reports that local commanders were feeding him. In any case the Qing had no coherent strategy for dealing with the British, which meant that the British also did not have any idea what they could expect from the Chinese, which led to them eventually just forcing a peace on the Qing when the hawkish Pottinger came along.
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As an example of how one-sided the 1st Opium War was, here's a paraphrased account for what happened when the Qing attacked the British-occupied Zhejiang coast on March 1842 (from "The Opium War" by Julia Lovell):
The plan of Commander Yijing was to have 36,000 soldiers attack Ningbo. In the event he could muster less than 8,000. Poor roads hampered the Qing advance and 50% of the porters assigned to draw the cannon deserted. The British had received so much intelligence regarding the impending attacking that they actually thought that the whole thing was a ruse. Yijing paid a spy within Ningbo to organize resistance but little came of it. Another spy was misdirected by Chinese locals and did not reach camp until after the battle was over.
The British had a couple hundred men to guard 3 miles of wall. At a section of the West Gate, 140 British were defending against around 3,000 Chinese. On 10 March, the Qing began with a water attack at 12:30am but only began their main assault at 4:00am. However, Yijing had ordered cannon and muskets to be used sparingly, so these sword-and-knife attacks were dispatched with 3 shots from a British howitzer. A supporting attack on Zhenhai similarly failed due to the British being forewarned, and another attack on Dinghai was never even undertaken because the commander did not dare attack.
At the point of crisis at Ningbo chief-of-staff Yu Buyun had to be carried from the battlefield because he had smoked too much opium and soon after decided to retreat. Following him, Yijing also abandoned his army and fled to Hangzhou ('in order to check the defences' as he put it to the Emperor), leaving 8,000 of his best troops stranded on the hill at Cixi. The 1,200-strong British force attacked the hill using field artillery, musket fire and bombardment from HMS Nemesis. 500 Chinese soldiers died and the rest surrendered for 3 British dead. By early April 3,000 of these soldiers were offering to become turncoats for the British.
But according to the report Yijing sent to Beijing, the Qing army had in fact stalled over 17,000 British soldiers, killing 500 of them including Lord Palmerston himself. As for the attack on Dinghai that never happened, the Qing apparently also managed to sink 5 men-of-war. The British evacuated Ningbo in May to support their assault on Nanking and Yijing reported that they did so because they were 'terrified at the advance of the Qing Army'.