Is there a word to replace the phrase for 'couldn't have been more different'?

I don't think the phrase is formal and academic enough, but I can't think of a word to replace it.

I want to say something like this: The two frogs both agreed that they needed to fight, but their fighting strategies "couldn't have been more different."

There are a number of modifiers to "different" which will work:

• completely different
• entirely different

etc.

Depending on the sense, you could go for opposite.

Another single word is diametric, one sense of which is completely opposed. Or, in that same vein, maybe antithetical, which is being in direct and unequivocal opposition.

Disparate

markedly distinct in quality or character

Merriam-Webster

(Similar to incongruous as aforementioned.)

• Antithetical?
• Contrasting?

Or perhaps similar phrases:

• The inverse?
• In stark contrast?
• Absolutely incompatible?
• Polar opposites?

Or for that matter, antipodean \ antipodal?