"If you play this stronger player for whom 'protection is sweet' you will be done for."
is my best attempt to understand. The context in which this happens is the teacher of this go club sits down next to this group of first years to play the strongest arrival to his go club, a first year who is in fact at a professional level and should not be there. This teacher is making an offhand comment to Uno and the other first years sitting nearby. If necessary here is the panel just before this.
So far I am getting the sense that が甘い can mean is relaxed and so on from the entries at eow.alc.co.jp, so perhaps "player for whom defense is easy?" but I am not sure.