Can't wait around for Brouhaha or Peasant Revolt army, as there's no guarantee you'll get the correct rumors within the quest time-frame. There are no big bandit parties roaming around (at least not anywhere near my kingdom). I doubt that disbanding most of my knights and then fighting a huge stack would be viable against Mystmountain Barbarians. Remember that I have 300 battle size and infinite reinforcement waves. This means against a larger stack, you are seriously outnumbered and can get completely swarmed (say 30 vs 270). I also play on slightly higher difficulty settings (~112% difficulty, normal damage/mixed combat/good ai/fastest speed) so even weak units can still deal a few points of damage here and there, which adds up quickly on your AI controlled units.
I actually took a party of 120 to fight a 700+ Vanskerry army and with perfect micromanagement and everything, I still went through my entire stack before bringing their numbers below 100. I ended up having to kill the remaining 60-70 Vanskerries by myself since I was the only one alive. So I got 55 renown for killing that entire army. Post battle my party was something like (9/97). Repeating that experience 5 times isn't really feasible.
With vanilla PoP settings and 150 battle size and normal reinforcement, this fight would've lasted multiple rounds in which I would've farmed ~40-55 renown per round for several rounds, before tapering off, for a net renown of easily 150-200+ That's why I'm considering 4x.