I CAN'T DO IT, I have tried with my two very amazing Creatures but they die half way through the battle and I can't can't can't beat the bandit leader what so ever and I have tried so many times and keep dying and dying. I hace used 10 potions, 3 revies and nothing still. Am I not doing something right???

I'm a little tired, so sorry if this is a bit short.

1.) Your first pet needs some updated gear (around level 20) and your second pet just needs gear in general.

2.) The first pet in your party usually takes the brunt of the damage in a battle and so should be statted to handle that. We call this pet a "Tank". Your wing pets if you have any should be your glass cannons, pets that are statted to deal damage.

Tank = Stat priority is Health and Agility
Glass Cannon (Magical) = Intelligence/Dexterity (with enough points in health to keep it alive)
Glass Cannon (Physical) = Strength/Dexterity (with enough points in health to keep it alive)

Also, a way I do it, considering all 3 of my Sylvesti are tanks, if one gets lower on health, I use the party management and switch them, and if possible, use mend, after switching them, I can use an elixir if needed.
Try to use "Finishing Blow" when you can, although make sure your Sylvesti have enough health to survive a few more hits to get mana to mend.

Spend all of your points if you have any left, and equip them with top gear, or they could lose.