Ah, yes, the eager young intellectual out to battle the demons of smug fuzzy-headedness here on the net. Well, Homey don't play that. We aren't going to argue with you, because we already know what arguments can and cannot be knocked down. As Kant pointed out quite a while back now, no metaphysical axiom can be proven to be necessary. Meaning, by its very nature, implies a metaphysical and teleological structure that is rooted in assumptions beyond mere matter. That is to say, you will be able to very easily poke holes in our presentation of the meaning of life, by definition. It wouldn't be the meaning of life if it were logically unassailable.
The only unassailable arguments these days are materialism and pure philosophical agnosticism. If you are dealing with someone who believes that the material world exists, you can win every argument by having the position that all that exists are atoms and molecules bouncing randomly around and that there is no moral or philosophical principle that can be proven to be true, or even to have any meaning. Life is completely pointless on a philosophical level, but if you want to continue filling your belly just for the sheer bloody- mindedness of it, Darwin pointed out the basic game plan and Ayn Rand filled in the egotistical details. Have at it.
If you have someone who is more clever and knows to argue that the material world may not exist, then there are not only no moral or philosophical principles that can be proven to exist, but indeed, there are no scientific principles that can be proven to exist either. You have sunken into solipsism, which of course can't be proven either, leaving you with no provable statements whatsoever. As before of course, if you wish to continue filling your non-existent belly with insubstantial morsels, there are plenty of other non-existent people who will apparently play that game with what we will for argument's sake call you, so jolly good luck, if there were such a thing.