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On a less tactical note, one thing I look for when writing a final climatic scene is how it can mirror the beginning. The beginning scenes of a book often foreshadow the end, and can give you ideas.

Since you’ve already decided that your ending will be a battle, you could look for emotional or character moments from the beginning that you can mirror in the final battle. If you start with a battle, you can put similar situations into the first and final battles. For example, in the first battle of one of my novels, the MC has to personally fight the enemy commander. In the final battle, she fights a general (albeit one more intimidating than the commander).

I’m not really an expert on battle strategies, so that’s how I look at things like that. Hope it’s helpful.