One way would be to point to the Blitz as a counter analogy, as the 'area bombing' of Germany was justified in part because of the indiscriminate bombing by the Luftwaffe of British cities like Coventry and London. This is useful because it invokes a cultural memory of having been the victims of urban bombardment, which would lead to an empathic reaction towards civilians in Gaza.
But I think a deeper, more pertinent response would be to look at the uses of 'area bombing' prior to WW2. Surprisingly enough this can be done through the career of one man: Arthur 'Bomber' Harris: https://en.wikipedia.org/wiki/Arthur_Harris And where did he develop the tactic? Iraq, India and... Palestine.
'In April 1920 Squadron Leader Harris was jointly appointed station commander of RAF Digby and commander of No. 3 Flying Training School RAF. He later served in different capacities in India, Mesopotamia and Persia. He said of his service in India that he first became involved in bombing during the usual annual North West Frontier tribesmen trouble. His squadron was equipped with poorly-maintained Bristol F.2 Fighter aircraft. In Mesopotamia he commanded a Vickers Vernon squadron. Harris later wrote of his time there that "We cut a hole in the nose and rigged up our own bomb racks and I turned those machines into the heaviest and best bombers in the command." Harris also contributed to the development of bombing techniques by promoting the deployment of delay-action bombs, which were subsequently utilised by RAF Iraq Command during their suppression of various Middle Eastern revolts (such as the Iraqi revolt of 1920, the Adwan Rebellion, the Ahmed Barzani revolt and the Mahmud Barzanji revolts). Harris once remarked that "the only thing the Arab understands is the heavy hand."
In 1936 Harris commented on the Palestinian Arab revolt that "one 250 lb. or 500 lb. bomb in each village that speaks out of turn" would satisfactorily solve the problem.'
This would pave the way for his later attitudes towards German civilians and the justification for visiting mass death on them for explicitly terrorist aims, which even the imperialist mass murderer Winston Churchill found distasteful (probably because it was aimed at white Europeans):
'Harris was just one of an influential group of high-ranking Allied air commanders who continued to believe that massive and sustained area bombing alone would force Germany to surrender. On a number of occasions he wrote to his superiors claiming the war would be over in a matter of months, first in August 1943 following the tremendous success of the Battle of Hamburg (codenamed Operation Gomorrah), when he assured the Chief of the Air Staff, Sir Charles Portal, that his force would be able "to produce in Germany by April 1st 1944 a state of devastation in which surrender is inevitable" and then again in January 1944. Winston Churchill continued to regard the area bombing strategy with distaste and official public statements maintained that Bomber Command was attacking only specific industrial and economic targets, with any civilian casualties or property damage being unintentional but unavoidable. In October 1943, emboldened by his success in Hamburg and increasingly irritated with Churchill's hesitance to endorse his tactics wholeheartedly, Harris urged the government to be honest with the public regarding the purpose of the bombing campaign,
"The aim of the Combined Bomber Offensive ... should be unambiguously stated [as] the destruction of German cities, the killing of German workers, and the disruption of civilised life throughout Germany ... the destruction of houses, public utilities, transport and lives, the creation of a refugee problem on an unprecedented scale, and the breakdown of morale both at home and at the battle fronts by fear of extended and intensified bombing, are accepted and intended aims of our bombing policy. They are not by-products of attempts to hit factories."
In his postwar memoirs Harris wrote, "In spite of all that happened at Hamburg, bombing proved a relatively humane method". His wartime views were expressed in an internal secret memo to the Air Ministry after the Dresden raid in February 1945
"I ... assume that the view under consideration is something like this: no doubt in the past we were justified in attacking German cities. But to do so was always repugnant and now that the Germans are beaten anyway we can properly abstain from proceeding with these attacks. This is a doctrine to which I could never subscribe. Attacks on cities like any other act of war are intolerable unless they are strategically justified. But they are strategically justified in so far as they tend to shorten the war and preserve the lives of Allied soldiers. To my mind we have absolutely no right to give them up unless it is certain that they will not have this effect. I do not personally regard the whole of the remaining cities of Germany as worth the bones of one British Grenadier."
So perhaps the best response would be to go one better with the guilt trip (while not accepting that ordinary citizens are responsible for the acts of their governments & militaries): - Yes, the British did bad things to Germans in WW2. They were responding in kind to prior German aggression, but they had also used this tactic in a similar way on the very people you are now targeting with even greater firepower. It was barbaric and inhuman when Hitler did it, it was barbaric and inhuman when Bomber Harris did it, and it's barbaric and inhuman when you're doing it. There's no acceptable justification for dropping high explosives on civilians. It's terrorism.