Why liquification of 0 group elements is most difficult?

There are many reasons

there is no free orbital available for bonding / sharing of electrons to take place ( since it has completed it’s octet)
Noble gases form weak van der walls force of interactions, hence to change their state from gas to liquid they need bondings higher than this van der walls forces.
they have a net repulsion taking place, since all the orbitals are essentially filled , they have the electrons which repel with the upcoming electrons
it is too heavy , so more force is needed to liquify it .