I don't know if BEK will out-do ninja on that team. He has two main purposes for CB - his aura, and his stun cleanse. Your original team had no stun cleanser, so I'm guessing someone else was eating them. Did you make sure to tune the team in a way that BEK goes *before* whoever is eating the stun? Or did you make BEK the lead, and thus he himself was taking stuns? If that's the case his utility almost completely vanishes.

For him to be useful - you probably want to have Skullcrusher lead, and BEK be slightly faster than him, so he can remove the stun.

at the moment I use tayrel as the stun target as tayrel's aura gives 25% def

If Tayrel is your stun target, and you want any real utility out of BEK, you need BEK to be faster than Tayrel. The real question, though, is why Tayrel is even on that team. He brings no value - draco and brago do everything he does. Also, with draco already there, I wonder whether BEK is blocking debuff slots by adding too many poisons.

he is there for dec atk

Brago brings DEF up and ATK down. Draco brings DEF down and Weaken. Tayrel contributes nothing, unfortunately.