There are two sides to defeating a politician like Trump. One, you need an alternative. Two, you need to explain to people why they shouldn't vote for Trump. All this is easier if your electoral system has mechanisms that require broad support rather than deep support. It's currently unclear if the Republican primary mechanism supports this. The delegate system does make it possible to forestall someone who doesn't win a majority of the delegates, but some people are actively opposing a contested convention.
In that specific case, I would look for people who fit Donald Trump's audience who nevertheless oppose him. Ask them why they don't support him. There are certainly plenty of reasons. Some examples:
Trump's Muslim ban was proposed in response to the San Bernardino shooting, but it doesn't actually address it. In response to criticism, he made exceptions for US citizens and family unification. Note that the San Bernardino terrorists were a US citizen and his wife (family unification). So the ban wouldn't apply to the actual terrorists. What's the point in that?
Trump says that he is going to protect American workers from free trade, but he sells suits made in China and Mexico. We need to elect Trump to protect us from ... Trump?
Trump would be the second successful businessman elected president on an anti-trade platform without ever holding other elective office. Who was the first? Herbert Hoover. How'd that work out?
Trump refuses to provide his tax returns. Every other candidate does this. What's he hiding?
Trump says that he gets audited every year. As far as I know, the only reason that the IRS audits people multiple years in a row is that they get money back every time. Why does someone as allegedly rich as Trump need to cheat on his taxes? Cheat may be a strong word here, but in political criticism it is often better to lean towards the clear exaggeration rather than the rambling measured truth. Why does Trump need to be so aggressive in claiming deductions is less powerful because it is harder to understand.
Trump says that he is handsome with great hair. If he's willing to lie about something so obviously untrue, what else is he lying about?
Trump's idea of a great Supreme Court nominee is his sister. Is the problem with politics that there isn't enough nepotism?
Trump says that he is now pro-life, but as recently as last year, he advocated nominating his pro-choice sister to the Supreme Court. Since the Supreme Court blocks more stringent abortion laws, what else is there? And of course, there are his older statements that he was pro-choice.
Trump says that he is going to protect American workers from competition from illegal immigrants, but just a few years ago, he was calling Romney's milder position "crazy" for chasing away Hispanics. Can we trust him to follow through now?
Trump is illegally misappropriating campaign funds to his businesses. His excuse is that he is funding his own campaign. But he's loaning funds to his campaign. So he could get paid back while president. If Barack Obama waits to indict him until after the Republican convention, what's the backup plan?
Trump's birth certificate says that he was born in Jamaica to a Scotswoman. What right does he have to argue that Ted Cruz and Barack Obama are not eligible to be president? Admittedly, the Jamaica is supposedly a neighborhood of Queens in New York City. But if the Dunhams could have faked a Hawaiian birth certificate even though Obama was born at sea (example Birther conspiracy), then why couldn't the much richer Trump family have done the same thing?
Trump claims not to be beholden to special interests because he is self-funding. But is he going to self-fund the whole billion dollars it will take to match Hillary Clinton (assuming her fundraising matches Obama's in 2012)? He says he only has $300 million in cash and securities. Where's he going to get the other $700 million? Loans? Sales of assets? If he's planning to sell $700 million worth of assets, he should have already started.
Trump is a special interest. He openly admits to donating to politicians to get them to help him.
Is Trump going to move his assets into a blind trust? If not, how will we know that he isn't going to use his power and influence to feather his own nest?
If we're critical of establishment politicians for being beholden to special interests, how much more critical should we be of Trump for being one of the special interests? He's the most establishment candidate, not the least. He's just cutting out the middle man.
Under his proposal, Social Security runs out of money in 2037. Either existing retirees will take large benefit cuts or taxes will increase sharply. Shouldn't we plan now to forestall sudden changes then?
He proposes increasing spending and cutting revenues. Won't that balloon the deficit?
This is not an exhaustive list.
Why haven't previous attacks worked?
It sometimes seems like criticism of their policies as repugnant, or the factual accuracy of their statements, either doesn't do anything, or makes them more popular, possibly because of a strong distrust of mainstream politicians and mainstream media.
Trump is often described as a populist. This is because he takes positions that are popular. Criticizing popular positions as repugnant or untrue will always backfire, because you are telling the people holding those positions that they are repugnant or wrong. Wrong can be a little better than repugnant, but neither is particularly effective.
The secret is to instead give people more information about the candidate. Point out hypocrisy. Give people information about the candidate that they don't know. Spend less time on telling people how to think about the facts they already have and more time on giving people new facts.
Some attacks will be more effective than others. While it makes for good comedy, talking about Trump's stubby body parts doesn't make him look unpresidential so much as the person saying it. At best, Trump's inadequacy about the issue makes him look weak. Leave those kind of attacks to late night talk shows. Concentrate on the more effective lines of attack.
The cheapest way to attack is to get the media to do it for you. But this is the hardest way to make an oblique attack. If it's really important to get the word out, put up your own money and buy time in relevant television time slots.
Divide and conquer. The best person to drive people away from Trump is Trump. How many Hispanics will vote Trump in November? Give him room to attack groups that show some support for him by criticizing him for not supporting positions that they hold.
Another big problem is that many of the attacks used on Trump have been used previously. Wasting accusations of xenophobic fascism on Reagan and Bush that would better fit Trump's stated policies has led to people discounting such charges. You can only cry wolf so many times before they start ignoring you. And then the real wolf attacks.
Changing the system
Part of the problem in the US is the archaic election system. Rather than ranking multiple candidates, voters can only choose one. There are some vestiges of an older system where delegates chose the candidates, but this is unpopular. So bring back the good parts of the system by letting voters express their preferences in rankings.
Such a system would allow candidates with broad but shallow support to show up better. Assuming that opinion polls would follow, this would have likely winnowed out the field quicker. If most everyone prefers Marco Rubio to Jeb Bush, then there would be little reason for a Bush candidacy.
Another issue is the artificial separation of the candidates during the primary. Some people have had to choose between voting in the Republican or the Democratic primaries. This has led to some wild swings as people who preferred Hillary Clinton might vote for John Kasich or Marco Rubio over Trump. Also, this has led to minimal interaction between Trump and the candidates most critical of him.
A non-partisan primary with ranked voting could better handle candidates with more moderate views. They could draw support from both sides. In the current system, people may have to change their party registration to vote for their favorite candidate. And what if they also want to vote in another primary? Why should people have to choose between voting for their favorite presidential and senate candidates?
The current system accentuates partisanship rather than encouraging candidates to rise above it.
I'm minimally familiar with Australian politics, so I won't try to comment on candidates nor the election system. I barely know that they already have some sort of ranked voting (STV? IRV?).