Not exactly sure how to prepare for something like this. Underground, but the Earth will be shaking. Caves are thus absolutely dangerous too. The surface will be unlivable for a while. Much of it may burn. Ice age immediately follows. A super hardened structure buried just below the surface can avoid the winds, and the water at high enough altitudes. Perhaps under water is safer, with massive tidal waves also expected, (the spin of the continents slows as magma plasticity increases, setting of all kinds of tectonic activity planet wide, but the spin of the oceans does not slow, they just keep going at their current speed, roughly 1000 mph at the equator.) Probably some kind of Ark. A really big one. With lots of animals. Maybe some plants.