To deal with the elemental ninjas, I used GNW TenTen + Mabui + Mei + Azure. Made clearing the plot instances pretty easy. If you don't have TenTen, you can buy her for 1200 coupons. Just having only TenTen is still possible, because she can still deal with all those lame element puppets.
For the actual tendos, you have to build a team that focuses on damage so you can take on down fast and possibly heal up the next turn while taking out another one the next turn as well. Sweep ninjas might help, they can attack all three and possibly getting it done faster, but mainly Chojuro is the one you'll need to sweep. I mainly used Chojuro to deal with them.