Even if you had a whole team consisted of Lv100 pokemon, it still won't lose. And no the rom won't freeze either if by some impossible miracle its taken down to 0 HP. I don't want to spoil it. You'll have to find out for yourself what I mean. My advice is, if you want to waste all your items trying to beat the thing. Have 2 seperate save files so you won't waste your money after you see it can't be beat lol.
About the Zekrom/Reshiram sprites, I haven't inserted them yet but their names are in Yape already. Their sprites will be in Beta2, which is when you'll be able to use them. Legendaries are recommended as the last part of the game will consist of 6 battles back to back without a poke center or mart. The 6 trainers will have very powerful pokemon, some exclusive just to them.