Play as Inferno.

It seems to me that taking a Castle from such an Elf is suicide.
....
How is it even possible to play like that?

It depends on which Elf... If he has antimagic and a spell that removes enchantments with light, then massiveness won't help much.
I'm saying, we play, seeing what the other person will do. We don't even let a mentor, even if there is one, get too much practice.
The most important thing in the game, even in HotS, is not the game itself, who wins or defeats, but how we communicate or "how did you manage to win?", or show friends a mega-build, something unconventional... Or they surprise me in the game... etc. The main thing is positive emotions... Playing online doesn't give me that. Why play online if you can play HotS well? Yes, sometimes we play online, but in the same room.)
Since I play HotS, my tactics are different... IMHO, it's not very logical to argue.

-1 speed won't help, because there are boots of speed and an aura of speed.

You should read my post in its entirety, where this is mentioned. Storm Wind doesn't do much on its own, but if it also has -1... then... draw your own conclusions.

And regarding the fact that you need to invest in Light just to reduce the speed of Warriors and Nymphs - it seems to me that this is a bad idea.

Where did I say that? Is there really a shortage of skills in Light? Light with Castles is good for Inferno, especially in the late game, where spells are unlikely to be a problem.

In any case, besides Nimue, I don't see any other candidates.

I don't see anyone at all.

And can you even play online?

I'm not going to and I didn't plan to. As I mentioned, the result in the game is not the most important thing for me, although I do want to win :)