I love those kinda fights to where I'm the underdog! You have to carefully plan out your next move, figure out which side to engage the enemy, and using the map to your advantage to come up on top. It feels very satisfying... Once you have those tactics down I actually feel like some battles are too easy.

Tips for battle to help you out:

1: Don't waste AP.
Let the enemy come to you! Don't waste AP walking to them... use corner to block line of sight and let them walk over to you.

2: Use terrain to your advantage.
Use sneak to scout out the area and pick the best possible position to engage the enemy.

3: Summons.
Make sure your casters have summons! They are very good "meat shields" to soak up most of the damage.

4: Abilities:
Don't just waste ability. I mean that by don't just simply pick an ability and click an enemy then boom next turn... have a plan and think a little ahead.
For example.. I initiate fights with rain and just on the edge of the puddle I created I would set up my summon ahead of my party, then most melee enemies would go after that particular summon and once they are in the puddle... I'd electrify it. In the early game my summons took all the beating because I engaged the enemy to where I want them and create situation that they would either have to go all the way around and waste AP to get to me or attack my summon or tank first.

5: Combining elements...it's very OP and can destroy enemies very easily. Like my boulder throw ability, it leaves behind a pool of poison then I would cast fireball to it and destroy everyone in that area... and if the enemy is way too spread out... I have two casters with the spell teleport..I'd bunch em up to one area and spread poison and fire.