Avoiding "existential it" while referring to a past event?

I know the use of "existential it" is frowned upon, but I'm not entirely sure how to rephrase the following sentence to remove it:

It is hard to tell what would have occurred if the battle had been lost.

Is something like this really all that bad? How can I rephrase this to remove that "it"?

That sentence is not using the "existential 'it'" that's frowned upon; it's just using an ordinary, unexceptionable feature of English grammar.

The "existential 'it'" that's frowned upon is the it that can be replaced by there; see e.g. http://www.odlt.org/ballast/existential_it.html, which gives the example of "It was nothing I could do" meaning "there was nothing I could do." But obviously your sentence cannot be changed to

*There is hard to tell what would have occurred if the battle had been lost.

A sentence that starts with

It is hard to tell

and continues with a tensed embedded question object complement clause like

what would have occurred if [something ...]

is an example of what's called Extraposition in English. Extraposition is a construction that takes a sentence with a "heavy" subject complement clause, like

What would have occurred if [something ...] is hard to tell.

and "moves" the subject clause to the end of the sentence (where it's more comfortable, since English is right-branching), and leaves a dummy it behind to fool us into thinking that the sentence still starts with a noun phrase subject.

It's hard to tell what would have occurred if [something ...].

Extraposition is one of many "movement" rules that place long complex constructions towards the end of the sentence, where they are more easily parsed and understood.

English Rule No.1 is

Every sentence must have a subject NP.

That's why there's a there in There's nothing here, and why there's an it in It's raining, and It's a long way to Tipperary, and It's hard to tell what else.

If you really insist on eliminating the it:

What would have occurred if the battle had been lost is hard to tell.

But there really is nothing wrong with your sentence. This is really a dummy pronoun, which is a common feature of English. E.g.:

It's a boy! [Said by a proud new father. Notice that he doesn't say He's a boy.]

It's sunny.

There is a cat.

It's too early!