I beat the Duo using liger's DmK strategy, I ended up with an extra potion and my Seaweed left over. Stacking nature resist is absolutely essential though, I picked up the following equipment pretty easily and finished with 53 nature resist.
Cute, Cute Queen Ring
BraveSirRobins Fearless Helm (you won't be able to get this unless you have the storybook since it comes from Chapter 12 Mogloween)
Leorilla Mane Belt IV (I'm assuming that if you're taking on the duo, then you've probably beaten Leorilla and Bearginion, so you probably already have this)
P.S. If you haven't beaten Leorilla solo yet, Riftwalker with BoA showing got me through him first try with no problems. Just cycle through Final, Double, Triple, and Power. As long as you only use those skills, he'll never put you in any danger. Since Riftwalker hits so many times per turn, the healing from BoA kept me above 75% pretty much the whole battle.