I've created a monster with three weaknesses:
A soft spot in his belly that can be pierced through normally with objects, but the monster is so big and fast that hitting it is extremely hard and dangerous.
The monster has a severe allergy to some types of coffee, to the point where surrounding yourself with coffee grains can keep him away.
I'm having trouble finding an explanation for the third weakness: a specific type of wood that can pierce through his armor even though bullets can't. Can someone please help me with an explanation about how a piece of specific wood could pierce through armor even when other harder, stronger materials can't?
Thank you all for all the answers, ive selected the most upvoted one as the "solved" one but you all helped me greatly! Thanks All.
The way i solved it is mixing all the answers in one explanation: Basically the monster has a combination of a non newtonian type of blubber (putting together Arcanist Lupus and Greig answers) that usually is strong against bullet-like projectiles and other blunt force stuff and also has a specific type of coat that is fibrous like a doormat (as chasly from UK pointed out) this way the monster gets pratically impervious to damage unless you use the right type of wood that generates the reaction Arcanist Lupus suggested. Also Cyn's response was extremely usefull for the purpose of getting the type of wood and helping in a way to mix wood with bullets.
Thank you all!