**WARNING** I have not actually beaten the gym with this set, however, I know it can be done at the right level, as I normally get to round 14 or 15, it's just the Venusaur's tank too much.
I personally think this team is the easiest to obtain and the most likely to survive. Simply bring 6 Wigglytuff's with Rest and Blizzard (or any other Ice type move, you want to freeze them.) You can do whatever you want with the other two slots. Your team should at least be level 60 before attempting this, it might be possible to do it at level 50, but you need the reflexes of a god to pull this off.
You first want to try to run through the gym yourself, to get a feel for it. Secondly, see where the waves come from so you are able to set up your team accordingly. Third, DO NOT FIGHT CELEBI!!! This is an instant death sentence, as Celebi can easily beat multiple Tuff's with one Solarbeam. You are protecting 10 candies, and only 8 Celebi appear throughout this gym, so you have little room for mistake. Fourth, the reason you need six Tuff's is so you can switch them out to Rest hurt ones while still getting a steady stream of damage. If you find a way to get around this, you can change the team, I just use Wigglytuff's for their bulk and relative ease to find.
*QUICK TIP: Route 5 is an easy way to find both Jigglypuff and money, just not EXP., use route 8 for that.