Question: My question is on 1st Samuel 17:40. Why did David pick five smooth stones when he knew one would do it? Also when good angels battle the evil angels, how do they fight and with what weapons?
Response: I'm not at all sure that David knew that he would put down Goliath with the first stone. David was clearly confident that the Lord was going to give him the victory (cf. verse 46), yet he prepared for battle reasonably. There is an important lesson here for us in applying our faith to this life we live. David didn't expect God to hit Goliath with a lightning bolt (though He certainly could have), nor did he doubt that God was going to bring victory through his human hand. David neither under-prepared nor over-prepared. Five stones would be enough given his level of skill (and he knew his own skill very well). My guess is that David was confident of hitting the Philistine with the first blow, but not so arrogant as to fail to take into account life's numerous possibilities: he might miss once or twice; it might take a couple of hits to bring Goliath down; one more for good measure, and that was plenty. David was confident, but not over-confident; he was prudent, but not in any way weak in faith. This, is seems to me, is the way all believers should approach all issues of faith, doing what we reasonably can and allowing for the fact that we don't know everything on the one hand, yet being entirely confident of God's deliverance of us and of His answering of our prayers on the other.
The five stones are important, because they let us know with certainty that we can and should do our part in any all things in which we are trusting in God, and should do so diligently and thoroughly, in the full understanding that such "due diligence" is not to be taken as an indication of lack of faith on our part, but merely the proper, respectful and humble response to God's faithfulness through an understanding that His ways are truly "untrackable" and it is we who must be prepared to adjust to His solutions, not the other way around. True faith trusts in God's deliverance without getting hung up on the specific means, or specific time of that deliverance.
As to your question on angelic warfare, this is a difficult one to answer because, as far as I am aware, scripture does not give us any definite information (see The Satanic Rebellion: Part 4, "Satan's World System", section III.6, "Angelic Combat"). Angels are spiritual beings, so it would make sense, from my perspective, if they did not use material type implements, like weapons, to fight each other (these would seem to be of the human realm). Still, the twenty four elders do have crowns, so we can't necessarily rule out material (or material-like) paraphernalia (cf. Rev.9).
It should be noted in this context that the result of angelic warfare is never wounding or killing, but rather only hindering (Dan.8), imprisoning (in the Abyss) or forcing out of heaven (Rev.12). In Rev.20:2, Satan is said to be "seized" and thrown into the Abyss, and it would certainly be consistent with what we are told about angels is their combat was of this nature (i.e., "hand to hand" so to speak).
Please also see:
Faith: the Means of Trusting God
Faith in the midst of the fiery trial.
The Faith Struggle
Strangers in the Devil's Realm
Hope this helps,
Yours in Him,