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.
-
The covenant would wreck the first order. A small group of rebels could take out their main base, so I'm pretty sure a large well trained army could too