@GumNaam has a valid point. Google 'Drone Swarm' you'll see. These are tiny drones and can be used to saturate the first layer very easily. They don't even have to strike the RADAR itself. Just need to explode EM bombs close by in large numbers.
Now consider larger drones that carry their own drone swarms with them and can detect a radar lock. As soon as that happens, they simply need to release their drone swarms. There might be holes in the idea, but it is a reasonable basis on which you can build something sophisticated.
But we need to look at this in a larger perspective. Pakistan's response to Cold Start was Nasr. Unfortunately, as soon as you use Nasr, India get's the right of second strike. And here is where things get complicated. The S-400 hampers Pakistan's ability to quickly attack Indian targets in a nuclear exchange. India effectively gains first strike by sacrificing their 'cold start' batallions. That's because the whole 'drone swarm' will take time to penetrate the defences. What we need is a method to take out the SAM sites very quickly, or something that sneaks past them, or something so fast that it is immune to them.
There are other options as well. If Pakistan is cornered like this, it can be forced to cause a mass electromagnetic attack using a large nuke. Google for 'High-altitude nuclear explosion'. And that is the problem of these advanced systems. Russia has done a disservice to the entire world, because it has just raised the stakes in a battle amongst nuclear states where one is beaten into a corner and will be willing to use anything and everything available. Think about the consequences of such an action. The EM fallout will NOT be limited to India only. A large enough explosion will affect Pakistan, India, Iran, Afghanistan, and most importantly China. If China finds this impedes it significantly, it can announce that any Cold Start by India will lead to simultaneous space explosions by China and Pakistan which ensure that both America and India get affected. The situation can get totally out of hand.
The S-400 is one toy which was not meant for children 5 years and younger (read Hindu isolationist extremists).