Because you only have Kaim and Sarah this battle can be a little difficult. The Armored Train has an ability called "Protection" that will reduce all physical damage to zero for several turns. In other words Kaim will become useless. Thankfully, it can't block magic attacks so have Sarah use Winda or Aquara each turn. If you see her go below 400 HP, have her use Heala on herself, because Magic Cannon will probably hit both of your party members for 300-350 damage. While Protection is up ,Kaim can't do much damage, so have him heal or buff until it wears off, then have him use Combo. If you're lucky you'll get a couple of hits in with it before the Armored Train uses Protection again. Spells and Combos will only do around 600 damage. If the train uses protection again, just go back to having Kaim heal and buff. Eventually the train will go down.