Well, I've kinda indirectly made it my goal to find a way to make any kind of Pokemon useful in its own right, because not every Pokemon can shine in 6v6 standard play (or any kind of metagame where most everything's allowed). One way to make to make weaker Pokemon more useful is of course to just play in the lower tiers (FU I'm lookin at you), but I've found that in-game challenges are the best way to let weaker Pokemon shine.

The reason I say that is that you can always make a challenge where you can make it work and still make it challenging, all you have to do is adjust according to which Pokemon you're using and which rules you go by. For example, if your skill level isn't too high, you can adjust the rules to be a bit slack (not using held/x items or the like), and if you like more of a challenge, you can just add stricter rules (a personal favorite of mine was going through the game without using damaging (aka physical/special) moves, not using any items in battle and not evolving my Pokemon link if you want to see a video of it :^) ).

To me, Pokemon is all about using your favorite Pokemon and having fun doing so, it just takes a lil time to find the arena where they truly shine among their peers.