To be fair a single wall titan is not a big deal in the OPM world. They are huge but Titan are quite light compared to their size.
A guy like Genos has the speed to maybe kill hundreds of them in a short time. But if they really are 10 millions he won't be enough to save everyone and I don't think he can stop the rumblings in all cities at the same time. So countries will be stomped.
There is also Eren. We don't really know his real power now : if he can grow a body with armor or if he can make walls etc.
But in a 1 vs 1 a class S can do it.
To really stop the rumbling all at once with the minimum of casualties Tatsumaki or Metal kight will be perfect with hundred of robots or huge aoe. But that doesn't mean that in 1 vs 1 a class S is needed to dust on big titan.
A guy like Stinger can kill a colossal without too much trouble.
The real difficulty is the number of those things.