That's a challenge, but not impossible.
I would use a modified version of the C.S. Lewis approach from the early chapters of Mere Christianity. I would demonstrate that apart from culture, evil exists. Start with the educated, industrial cultures -- and was there ever a better-educated culture than Nazi Germany? Was there ever a more affluent culture than ours? Yet immorality is epidemic. Then examine the idea of the noble savage -- is it western culture that leads to immorality? But theres a reason why the term "savage" was around -- because they were no better.
CS Lewis argues that we betray our knowledge of morality when we object that we have been wronged, but try to demonstrate why, in this particular situation, we are some kind of a special case. "If you only knew what kind of a witch my wife is, you'd have an affair too." That kind of thing. That we consistently fail to live up to our own moral standards. This is the essence of the arguments of Romans 2.
From there, you need to demonstrate that we're completely selfish. Not hard.
Then you've proved total depravity.
You mean, "subjectivism," as opposed to, "rational self-interest"? Right! (I hope that's what you meant, unless you're an altruist.)
Hank