The best defense is a good offense.
You, my friend, have fallen into the anti's trap of playing defense and trying to justify yourself and your "need". Don't. Make them defend their position instead. Why do you "need" an assault weapon? You don't, and you can admit you merely "want" one. There's many things we want but don't need. No one needs an SUV the size of a main battle tank, but soccer moms all over the country are driving them while blabbering on the cell phones they don't "need". No one "needs" a 36" TV, but we do like having them. Similarly, you don't "need" an assault weapon, but you'd like to have one. (And it should also be noted that you don't "need" the right to worship or speak as you please - many countries don't allow you these freedoms- but we as Americans like our Bill of Rights and our Constitution and the rights and freedooms enumerated therein.)
The Constitution enumerated certain rights; the right to keep & bear arms among them. Those rights can be restricted and regulated (and if you don't believe me, we can get into a lengthy discussion of legislation under the Commerce Clause and its Constitutionality as applied to a right enumerated in BoR). Currently though, the law of the land allows us to buy and sell them. So you "want" an assault rifle to exercise your God-given and Constitutionally recognized rights.
The usual counter of the anti is that the framers of the Constitution didn't have modern assault rifles in mind when they wrote the Constitution and didn't intend to protect such things. Maybe, but the Constitution is a living document and we apply the principles embodied therein to modern times. The internet, broadcast TV & radio, and the mass media were not in existence when the Constitution was written, but we do apply the principles of the First Amendment to modern things such as the internet. Ask them if they'd like the first Amendment's free speech protections to only apply to materials written on antique printing presses or things writen with a quill on parchment.
Then they will counter with the old cliche that someone may use it for something criminal. We already have laws regarding murder, assault, robbery, etc. Merely possessing a gun does not harm anyone. If someone were to use it in a criminal fashion they've crossed a line. If it gets stolen and used in a crime, the criminal has alrewady shown his disdain for the rule of law by stealing in the first place. Would you think he would care about laws proscribing his possession of an assault weapon if they were outlawed? Should we outlaw possession of certain common household chemicals because someone might use them to make an explosive device? Should we outlaw possession of gasoline outside a vehicle's gas tank because someone might use it to make a molotov cocktail?
Ask them to provide the number of federal criminal convictions under the 1994 Assault weapons ban that was suppossed to be so damn successful that it desperately needed to be renewed. (The fact is, assault weapons as defined under federal law are used only in about 3% of all gun crimes).
Ask them to define an assault weapon which they would ban, and then point to examples of commonly used hunting and sport rifles that use the same action which would also be banned. Blur the already vague lines between "assault weapons" and hunting rifles and make the anti's try to distinguish one from another and where the line between assault weapon and hunting rifles is. In 1994 Congress and the US Senate had a very difficult job of it (which is why the law was mostly unsuccessful); my guess is that your anti friends will to.
I could write more, but I think you can see where I'm going...