Magic Guard Clefable is far from being one of the best Pokémon to beat stall with, it has been used for that purpose for a long time and though that wouldn't happen if it didn't have some very useful tools at its disposal, it won't overcome any stall team made by an experienced player.
Firstly, even at +6 it fails to 2HKO Chansey (and Unaware Quagsire/Clefable), so between them they can stall it for 32 turns+ with the combined recovery of Soft-Boiled and Recover (with Wish the stalling is even easier), and Quagsire can come in when it likes to remove the boosts with Haze if another team member is better placed to defeat it. Amoonguss can remove its boosts easily with Clear Smog and waste its attacks' PP by switching out for Regenerator recovery. Any Pokémon with a fast Encore will cripple it, as will any Pokémon with Trick (Trick Gothitelle is finding its way onto a lot of stall teams). Switching into it with a fast, bulky Taunt user also completely shuts it down - Mew, Heatran and others can do this very well. Unaware CM Clefable beats its Magic Guard counterpart every time. All of this goes without mentioning the fact that many stall teams do have a Pokémon on them with the power to muscle through Clefable before it can boost too highly.
Speaking from experience, Cosmic Power Clefable can work. You just need to give it some support, i.e. clear out the opponent's obvious counters with trappers. I made a very successful team with that strategy (1800+).
Also, Chansey is great choice for balanced teams. I might write up a proper post on this later, but it is quite possibly the best Pokémon to use as a general sponge for incoming strong attacks. Along with outright walling too many Pokémon to count, and absorbing & clearing status, it can survive almost any boosted hit in order to cripple something with Thunder Wave, saving a team from being swept. People love to use Thundurus as their anti-sweep backup plan, but Chansey does a perfectly fine job of it too.
+1 252+ Atk Tough Claws Mega Charizard X Flare Blitz vs. 252 HP / 252+ Def Eviolite Chansey: 570-672 (80.9 - 95.4%) -- guaranteed 2HKO
Seriously one of the absolute best Pokémon in the game