It shouldn't be too hard to beat them, but here is a guide for it anyways...
The Team Preview
The main Warlord, [Shingen], has the warrior skill Rally (this raises the Pokemon's and its adjacent teammates' Attack by 1 stage) along with his Pokemon partner, Rhyperior (the ability is Lightning Rod). Shigen is also holding a [Freeze Block].
The other two Warlords, [Yukimura] and [Kunoichi], have Tepig (the ability is Blaze) and Scraggy (the ability is Shed Skin) as their partners (respectively). Yukimura can boost Tepig's attack by 2 stages with his warrior skill Mighty Blow, while Kunoichi can give gradual recovery to Scraggy and its adjacent teammates. Yukimura has a [Power Band], while Kunoichi has a [Potion].
The other three are just regular warriors. One of them has a Rhydon (the ability is Lightning Rod) with the warrior skill Mighty Blow. Another has a Drilbur (the ability is Sandpit) and the warrior skill Brotherhood. Finally, the last one has a Charmander (the ability is Blaze) and the warrior skill Adrenaline.
Preparing for Battle!
Obviously, Water-types and Grass-types have the clear advantage, so you should recruit some warriors in Fountaine. Ground-types here also are okay (if you can even get any).
You can also use the code for recruiting the warlords of both Fountaine and Greenleaf, which is J2TRZXPUm3. I think you open the menu and press Codes, then you can enter this event code (which has no expiration date).
Also, if you recruited Yoshihiro and / or any Fighting-type warriors from Pugilis, they'd also help alot.
After that, you can then grind up your Pokemon up to a decent links (I'd say anywhere between 45% - 55% is good).
You can reclaim your kingdom Chrysalia (the Bug-type Kingdom) by challenging and beating them. Just use the same tactics you used to beat Yoshimoto (the original Warlord) and you'll be fine. Then you can finally challenge them at Terrera. If you don't get your 'special reward' (I don't want to spoil it for you) before challenging them, it looks like you'd have to challenge them again and beat them at your kingdom.
After your 'special reward', you could continue grinding or challenge Terrera.
When you do challenge the kingdom, you'll see that this field is pretty simple. Do not send your Pokemon up the tower until turn 3 because it will always blow when the first Pokemon reaches the tower. It'll do damage and knock the Pokemon off. Afterwards, it will blow at random times.
A Pokemon you should worry about is the Drilbur. With the Dig attack, it would temporarily "disappear" on the first turn, then attack on the second. When it targets you for the attack, it can still hit you even though moved out of its range. However, Drilbur would end its turn right after using it (and it would be on the same square when it used the attack). The trick is to attack it after it finished using Dig.
The only other one you need to deal with is the Rhyperior. It has a strong attack, but on the turn after it attacked, it has to 'recharge'. You can catch it while it's recharging and go for the potential OHKO.
After Turn 3, start sending your Pokemon up the towers and claim them. Have your heavy hitters (Grass-types, Water-types, and Fighting-types) target Rhyperior, then Drilbur, and then the other Pokemon.
Well, I hoped this guide helped you! Good luck on conquering Terrera!
Source: Above links, Past experience