If you can replace the word with “he”, you always use who. If you can replace the word with “him”, you can use whom if you want to.

Whom did you lead into battle?

I led him into battle.

Who ate all the cake?

He ate all the cake.

The key takeaway is you can always use who and it will be correct, because who is both a subject and an object. So, if you don’t want to bother with the rule, just stick to who and you can’t go wrong.