Help with completing the game.

I can't kill the slimes in the Dragon Cave.

Slimes can only be killed with magic. Since your squad is not magical, and leveling up magic skills for archers is not particularly beneficial, take magic-casting wands and equip your guys with them. The slimes are immune to physical attacks. By the way, various NPCs mention this, so I advise you to read their dialogues more carefully – it's useful.