It's really hard to stop missiles. Most air defense systems do well against conventional aircraft but not so well against missiles, which are smaller and faster or, in the case of cruise missiles, follow an unpredictable path.
Not unlike what happened with Saudi Arabia getting pounded by missiles and rockets from a ragtag militia despite having the Patriot system.
There isn't much Russia can do itself outside of dispersing their forces and supplies. No country, even the US, would have enough conventional missiles to completely destroy such a large area, eventually conventional aircraft with cheaper, more numerous bombs, would be needed.
However, Russian casualties will be high regardless.
Their main problem is that they seem unable to destroy the vehicles/aircraft shooting these missiles. HIMARS isn't supposed to be some unstoppable force, it's still a truck, it leaves a giant smoke trail leading back to it. Yet, Russia isn't able to quickly dispatch a plane to destroy it.
In large part that's because they're afraid of being shot down by mostly Soviet era systems.
In other words, Russian/Soviet equipment is pretty good but Russians suck at using it.
