To get good results in our duels/battles is important to take some aspects into consideration, among them theres the conditions of the duel and our cards ability.
It isnt just about having the cards leveled up, which obviously gives them more attack and defense power, but to use the best combination for them.
Ill use a recent duel I had in which i turned out as the winner as an example. The duels condition was Super Sneak, all monsters with malee attack have the sneak ability. For this type of duel you can use Fire Splinters card but you can also use the Life Splinters ones. In Life Splinter we have a very good card for this kind of duel: Silvershield Knight, this card has the ability to inspire, with it you give all friendly monsters +1 of malee attack. This increase of malee makes it so that all cards have a much more efficient attack.
For the battle i used the Life Splinte, and i incluided in my alignment all the malee cards to be on help for the Silvershield Knight.
My opponent also used Life Splinter but his priority was to the range cards instead, which is a very effective combination but not for this type of duel. The result was that soon my malee cards attacked all of his background cards, destroying his last line alignment. In a few rounds my opponent had already lost, he had no defenses.
Thats why its so important to take note on the duels conditions and your cards ability,so that you can actually design a good game strategy.