Please move your cursor on the monster then the cursor turns into a sword - left click to enter the battle.
Setting up your four mostly used skills on the first four slots, 1-4, and fighting monsters manually is far more effective than allowing the computer to decide who it hits with the one move it will use. This is particularly potent when a group knows which enemy needs to be killed first as attacking one monster at a time in a co-ordinated way, giving them less attacks, reduces the damage received by the tank, lowering overall durability loss. Your inventory space will not need to be replenished as often and you will save all that gold.
During solo play, waiting a second or two and attacking the same monster as your pet decides to achieves the same result.
When you have enabled automatic battle, then the computer will decide which monsters to attack and as said above this is not always desirable, but sometimes you want the freedom or just dont care. In this case can often be wise to pick a default skill the computer must use, however note the computer can only use that skill. The same applies to your attack pet.
During an automatic battle the computer will also use your HP, MP and HP/MP potions when needed. When your HP or MP is below 20% of your maximum amount then the first item in your bags will be used, regardless of how much it restores. The computer searches row by row, bag by bag - beginning with first slot from the left on row1 in Bag1.
To maximize usage of potions/herbs and the like, should you place them in the order of smaller to bigger, but begin with those that restore close to 80% and save the smallest ones to between battles where do you not waste a battle turn, which you should use for attacking the monsters. Restored amounts that exceeds your maximum HP/MP is lost.
You are down to 2318 / 3321 HP and 100 / 600 MP in an automatic battle. The first item in your bags is "Delicious Crab Meat" which restores 1500HP and 200MP.
At your next battle turn the computer knows your MP is below 20% (16.67%), so it will use "Delicious Crab Meat" and restore you to 3321 / 3321 HP and 300 / 600 MP.
2318 + 1500 - 3321 = 497 HP restore points lost, which is ineffective usage but maybe acceptable.