Third sequential post, for which I apologize, but c'mon guys, grab this game, it's awesome!
Ok, I finally figured out what the requirements for recruiting Warlords are. Normal Warriors are recruited by killing them in less than 3 turns, killing them without taking damage, or killing them with a super-effective move.
Warlords have the same requirements, with the added one that the killing blow must come from another Warlord.
I like running a mostly Warlord team anyway. Unique portraits and personalities, and solid starter Pokemon. Since killing with super-effective attacks is the best way to recruit, the key then is to figure out which ones to take to which battle.
During the main story, you only get a handful at first. Your main character and Oichi to start out. The DLC event gives you two more, Grass and Water. You can grab another two from Violight, and this is probably the hardest recruitment of them all, since the only Pokemon you can get before that point that is supereffective against Electric Pokemon is Quagsire, and that takes a lot of grinding. But with a fast moving team, and taking advantage of Eevee's awesomeness (move again after killing something + extra range from your leader's ability), and you can get there.
At that point, things become a lot easier. Level up the Starly you got from Muneshige until it becomes Staravia. Congratulations, you have everything you need for Chrysalia and Pugilis. Staravia completely dominates those fights, especially Pugilis, since everything they have is weak to Flying, letting you recruit your Fighting and Bug Warlords with little trouble.
Things change a bit depending on which of the two areas you attack next. Illusio is the harder of the two (due to the stupid map), but I just love Psychic Pokemon so much that I go for it anyway (also, having Eevee do anything in Illusio while its Link is 70% or higher levels it into Espeon!). In which case, Pineco is your best friend (and, honestly, I find Pineco better than its evolution, Forretress, which has a really, really poor attack ability). Take your Fighting Warlord with you, just to give you a Warlord who can super-effective Aya's Ice type Cubchoo (in case you can't kill it in the recruitment turn limit), and either Dewott or Servile can thrash the Gound type on the team in probably one hit. Pineco cleans up the rest, and then you can eliminate Gallade at your leisure.
Just remember, Shingen joins after you beat Illusio, but his two follower Warlords become random encounters, so hunt them down and recruit them too (which, since they are on their own, will be simplicity itself).