First, you unlock Mr. Game & Watch. This can be done in a variety of ways, but my personal favorite is the Subspace Emissary. All you need to do is beat Duon. (You know, the guy that got its voice clips replaced with Roy's that one time for Project M? Yeah, that robot!...)
Then, practice all of his moves until you get a good feel for him. He has plenty of moves with great potential (his side B and down B are OHKOs), but for this part of the guide, we'll only be using two: his up B and down aerial. All you do for a simple combo is repeating those moves quickly. (The down aerial should be performed while your parachute is open.) That's the entire combo.
Now, about the offstage game...I never really focused on that, so I'll update this later.
Defense is one of the most important parts in any fighting game. Blocks, counters, airdodges, you know the drill. The problem being...G&W only has one counter, and it's only for projectiles. So what do we do?
Mr. Game & Watch has some of the best recovery options for characters that can only jump twice. trampoline goes pretty darn high for what it's worth. Went too high? The key can get you down to earth. Not in reach of the ledge? Speed matters. Facing a Meta Knight? Dinner at 9!
Follow this guide and you might beat someone. I don't know; I'm not very good either.