After spending way too much time trying to beat the Fire Elder and going through that freakin' water maze a hundred times, I decided to make this thread to help any of you who may also be having a hard time with this NPC Battle from the Legendary Bird Quest.
If you picked Torchic as your Hoenn starter you're in luck, as this is the Pokemon that will solo his entire team once set up properly! Otherwise you can buy one for pretty cheap off of another player. It does not need any specific IVs or Nature, however you will need to max the EVs in SpDef and HP because you better bet your boots that Torkoal, the first Pokemon on the Elder's team, will spam Earth Power as much as he can before you finish setting up!
You'll be wanting to have your Blaziken hold Leftovers to increase your odds but the most important here is the move-set. You absolutely need Rock Slide, Bulk Up, and Double Team which the last may be the most annoying to find. The fourth move is not important at all.
What you want to do is simply Double Team 6x in a row and then Bulk Up 6x, after that all that's left is to sweep his team with Rock Slide. You shouldn't run into any problems unless you miss your Attack and the opposing Pokemon manages to hit you even with all your increased evasiveness, or if you get poisoned while your health was lowered too much by your first fight against Torkoal.
Here's the Blaziken I used for reference:
Don't give up, Good luck!