I think the best way to debunk existentialism is to provide some small evidence of order in our universe. The most easily visible evidence of order is the existence of probability. A somewhat unclear explanation of this exists in God's Debris, by Scott Adams, but that book is choked with a bunch of fairly comical philosophy as well.
In terms of mathematical probability, almost every possible population on this earth can be found to exist the
characteristics of a normal distribution. In the case of the human race, we have some extremely good individuals and some extremely loathsome individuals. However, most people would fit into the bell curve that exists between the two extremes. This same representation would exist between the strong and the weak, or with any other conceivable, quantifiable population trait.
Here's a good question that harks back to the beginning of any statistical education. When you flip a coin an infinite number of times, the results will be very nearly equal to half heads and half tails. Why? If the universe is chaotic, shouldn't the possibility be the same for flipping all heads, all tails, or the 50-50 distribution? Now someone may counter argue with the fact that how do we know the results when it's not possible to flip a coin an infinite number of times. The simple explanation is that all evidence points to the existence of probability. The existence of probability, in turn, points to order. I'm not talking about blindly believing in the religion of probability. I am talking about overwhelming evidence towards the existence of such order.
To say there is no order in the universe is a cop out for those who realize they will never understand that order. I want to be clear that I do not believe we will ever truly understand the motive force behind this universe, but I will also not throw up my arms and surrender to chaos simply because I am unable to understand or comprehend such a thing.
Now the existence of probability very likely provides nothing as far as creeds by which to live life. That is the domain of religion, not science. However, to just throw up your arms and say there is no order is just as ignorant as blindly following an organized religion because it makes you feel good.