the evil team spawns camps faster

Hello, the chance to spawn an advance camp also depends on the faction's strength. So if you manage to focus on one faction at a time, you may be able to take them out for a while.

I think you have one path: focus on relentlessly crushing the weakest enemy faction until they're out of the war theatre. Then move on to the next.
You must suppress them one by one, without scattering your strength and attention.
Besides faction strength, the capabilities of your army are important. Beornings are cool but difficult! Consider the strengths and weaknesses of your troops against each enemy faction.

I'd raise armies of Beornings and Woodmen for swift, including night, engagements against Umbar and Mordor. Shield walls, for which dwarves are ideal, are crucial against heavy cavalry. Dale's armored men can withstand blows but suffer losses. A strong archer detachment – accurate elves or hardy Northmen – can halt enemy attacks even without melee. Massed cavalry overwhelms archers. I probably shouldn't have bothered with basic tactics, but just in case.

Okay and with a cynical and calculating tone:
And a bit on tactics and strategy, since I'm such a bore ;)
Support small scout teams after they're engaged. Let them initiate, then join. If you help them, you get rank and influence. If they help you, you get nothing but exp and wounds))).

Sorry if I'm stating the obvious. In this war, details matter. Strength can break, and weakness can become strength. ;)