Weyl semimetals (WSMs) are novel gapless topological states of matter with low energy electronic collective excitations behaving as Weyl fermions. WSM is the first experimental realisation of Weyl fermions in nature. Remarkably WSM can be used to test our understanding on quantum anomalies in quantum field theory and therefore it has attracted a lot of interest in last few years. In Weyl semimetals there exists a topological quantum phase transition from the topological nontrivial Weyl semimetal to a topological trivial semimetal or insulator phase. In this talk I will show that using the tools of holography it can be verified that the topological phase transition and the concept of topological semimetal obtained at the weak coupling limit in a Weyl semimetal still exist in the strong coupling limit. I also show that there is a new observational prediction for the mixed gauge gravitational anomaly: there exists substantial odd viscosity in the quantum critical region of a Weyl semimetal. We hope it can be confirmed in experiments.