Use logic, feats, and stats.
Assume firepower and shields are equalized.
The First Order has StarKiller.
The Covenant have High Charity.
The galaxy's are merged. Covenant on one side, First Oder at the other.
Both at their peaks.
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Sure, Starkiller Base can blow up planets from afar, but what's that next to the Halos? (And don't nitpick and say "b-but that's Forerunner" because we saw them take and nearly activate them several times. The Halos are essentially theirs.