One thing creationists often trot out is how improbable the universe is.

There's really two separate parts to this. One being how improbable it is that the world around us seemingly so perfectly fits human life. Creationists generally use the word "perfectly" but it's clear we're not perfectly suited to the Earth as a whole, a desert isn't a very nice place for a human for example.

The Earth doesn’t fit perfectly with life, life just managed to change enough to survive. Life on Earth evolved to suit the conditions life found itself in, it's like asking why a puddle of water fits so perfectly into the whole in the ground. That's just how it works. Life that breathes hydrogen in order to survive wouldn't last very long, yet life which breathes oxygen would survive as we now have oxygen in the atmosphere and so on. During mass extinction events, would they say the Earth fits life so perfectly? Of course not because a lot of species would be dying due to new conditions they find themselves in which they didn't evolve for. It's flawed reasoning to the very core.

The other addresses how improbable things are at the cosmological scale, i.e. the probability that matter could form or the universe wouldn't collapse an instant after the Big Bang.

On the cosmological scale and even on the life on Earth scale, you're doing probability backwards. Which simply doesn't work.

If you took a dice with a million sides and rolled a number say 159154. What were the odds of you rolling that number? A million to one. Yet it happened.

If we say you cannot exist unless that number is 754689 and the dice keeps rolling, eventually it'll hit 754689 and you'll exist and pop up and say wow how improbable this was. The fact the dice may of rolled a million times already, but because you didn't exist and therefore couldn't record those million attempts doesn't mean it's impossible; it was simply inevitable.

You can't do probability backwards, it just doesn't make sense.