Spending time on 'what ifs' is pretty much a waste, but, hey, needless speculation is really fun sometimes.
Well, Lothlorien had a lot of archers, and considering they were probably as good or better than Bard, Smaug would have been shot down pretty quickly. The Balrog would have been a bit more trouble, but the elves could swarm if they had to.
The newscast would be like this:
Dan Rathelf: "It's nothing like I've ever seen! I'm seeing a mad herd of elves trampling over a Balrog. And this just in, over at Pelennor Fields, we're receiving a report that at this same instant there is an army of undead warriors swarming a Mumakul! What a coincidence! This could be just the thing to change the outcome of these battles."
And that just leaves a bunch of pansy orcs to deal with. Childsplay!