Skull isn't weak in the first place; he just focuses on more defensive and durability-based attacks, so figuratively speaking, he is the weakest among them, as he has no attacking power. Other than him, it hasn't really been explained further who is the weakest Arcobaleno, so only Akira Amano knows.
but once reboorn stated that Verde uses his brains rather then fightng with his body. Also yuni doesn't fight and had never trained so she is not fit for training.
He is not the weakest.