The final battle quest as Beastmen is interesting. I feel like it's another thing all factions should have.
In general, raze almost everything for favour and only get a third herd when you're absolutely flush with favour and on the cusp of getting the 25/50% upkeep reductions on your other herds. Otherwise it will be unsustainable (if you're fielding high-tier armies, at least. Which you will want to, to stay competitive). Raid before you go all in and you can offset costs further and bleed out some extra income--for some areas, with a large army, it can be a cool 1k+ per turn; not to mention the herds you can attract, which will let you take out regional capitals.
The best places to get some quick favour is razing/raiding Greenskin lands near the coast, or Bretonnian/Estalian lands near the mountains; both will be largely busy elsewhere and will have good escape options (beast paths). Raiding specs for the lords are recommended--they will help a lot.
Also, I'd advise against fighting Dwarves. Their armies are a huge pain, due to all of the armour, charge resistance, and high leadership, and their cities are always highly entrenched and difficult to reach.
Now... as an aside, it actually doesn't make sense for beast paths to go over water as I understand it. But whatevs.