Probably the easiest way is to just battle them at castles. Wait for them to besiege you castle or city (preferably Forniron if you have it as it only has one ladder) and while they're weak raid theirs. It is incredibly difficult ot times to defeat them in a normal batle due to their swifthunters and keshings (and now heavy riders which I think are insanely good). The main way that I found to be easiest to defeat them on a normal battlefield is an archer army with like 1/3 of that being units with shields. As you may know the Elintor are like the best faction in the game other than the Realm of The Falcon and thier arrows will deal swift work of the Zann swifthunters and the Zann dont have good shieldmen. Sorry i made this so long but I too have been thinking about that for a while now.

Too long? Ha! Its nice to read once and a while, but still thanks and yes it is very hard at times to fight on land defending Castles/Citys seems to be the best way if you are a Archer/Crossbowmen/woman. My tip is to use the Commander perk of U " I think it's U" From my understanding it is more or less a healing Inspire that you can use every 60 seconds that and the Y heal all costs XP. U is around 60-70 and Y can be 500 yet fail at times and cost 4? or give 4? IDK I tend to get head shot before I can use it.