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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The first order, starkiller base would just blow up high charity. And with the covenant now being decentralised and with the loss of the majority of their political leadership the reaming military factions would devolve to in fighting.