Tentacruel seems really useless to me. The only set that I can see working is the speedy spiker set since it actually outruns pretty much everything that isn't scarfed in ubers, barring Darkrai, Skymin and Mewtwo, and sets up Toxic Spikes with ease. It can take people by surprise because a lot of players forget that Tentacruel has base 100 speed. I would honestly never use it but I remember mien getting far in a Smogon Tour with Tentacruel on his team, you could ask him about it if he's still around.