First, I don’t really see this as an argument against the truth of Christianity, more than just a disapproval of its teachings, and ultimately of Jesus’ sacrifice on the Cross. For all intent and purpose, you could grant the skeptics’ point, but simply point out yourself that that has no bearing on whether or not Christianity is true. Moreover, this charge of Christianity being weak or for the weak is probably one, if not the, oldest claim against Christianity, since the cross was, especially in Greco-Roman culture, truly a scandal.
Second, a prima facie justification for demonstrating that Christianity is not literally just for the weak would be to simply point to certain particular Christians, who themselves are anything but what the world would normally associate with physical or mental weakness. For example, when I got saved at the age of 34 I had just qualified for Army Special Forces (i.e. Green Berets), eventually spending about 17 months in what is called the “Q-course”. Ultimately, I withdrew and returned to my normal, “big army” job as a paratrooper and intelligence analyst for the 82 ABN Division. Now, this is just to set up some context. My first church at Fort Bragg was filled with military personnel. Because we were at Bragg most of the congregants were Paratroopers with the 82nd, but we also had several soldiers who were Green Berets or other Special Operators (Rangers, etc). My first two Bible teachers and mentors in the faith were both CAG (Combat Applications Group) operators (aka Delta Force) and were about as physically and mentally tough as any human being possibly could be. They were also devout Christians, deep lovers of Jesus and Scripture, and amazing family men. So right there one would have examples of Christians who for all intent an purposes were not weak in the traditional sense that the world would identify a weak person.
Now, it could be that the skeptic makes a more nuanced claim about weakness. For example, they could say that my friends were weak, not in that they lacked physical or mental prowess, but in the fact that they seemed to rely on God and Jesus in order to find the mental and emotional strength to carry out there difficult vocation. He might argue that an atheist could also be a Special Forces operator, do the same kind of things that my friends did, and be just as proficient in them, yet without relying on God for mental and emotional strength. But here I think the skeptic fails to understand how God operates in the world and how we experience God’s operation in the world. Certainly an atheist could be just as physically and mentally strong as my Christian special forces friends. Certainly there are atheists who actually are in Special Forces positions like this, so counterexamples could be easily produced. However, I don’t think that the Christian SF soldier thinks or believes that God actually miracles them to be physically and mentally strong; i.e. it is not as if they don’t need to train and prepare and work hard to be a SF soldier. God is not the efficient cause (or the proximate cause) in their actual regiment of becoming a soldier. They have to do exactly the same things as the atheist SF soldier, and the experiences of doing those things is essentially the same since God operates through natural or secondary causes.
My point is to say that the atheist SF soldier and the Christian SF soldier are psychologically about the same when it comes to mental toughness and mental acumen with regards to the rigor of their jobs. Physically they train in the same way, and for the most part there is no discernible difference between their experience of the physical and mental rigors involved in their occupation since both are working through God’s secondary causation (i.e. it is not as if the Christian SF soldier can laze around on the couch all day and eat Twinkies because God will miracle him up the obstacle course, or miraculously make all of his bullets hit their target). Thus, I think simply pointing out that there are such Christians is prima facie evidence against the claim that Christianity is for the “weak.”
That said, I think there is at least one discernible difference that may obtain between the atheist SF soldier and the Christian SF soldier. The Christian SF soldier may be in the position to have a greater capacity for the virtue of gratitude, since the Christian will see his own powers as derivate from God. He does not experience God as miraculously accomplishing his difficult tasks for him, but he does see his own capacities and capabilities as unique endowments from God, and thus he can praise God for these gifts, whereas the atheist could not.
Finally, from a biblical perspective, one could also just show the skeptic Rev. 19:11 and demonstrate that while it is the Christians role to suffer in this world as Christ suffered for it, that that will not always be the case. I know some people take Rev 19 to be just symbolic, and I would say “okay” to them, but it is symbolic for something and that namely for Jesus eliminating evil and specially Satan and evil people with dramatic and swift violence. Thus, while the Christian may be called to weakness or mildness in this life, it will not always be the case. There will be a time for the righteous to reign and rule; but that time only comes once love is perfected through our life in Christ. You could ask you friend if he really would want that unloving and self-centered people to be in positions of power? Sounds like Ayn Rand or something if he were to say yes.