I'm new to the idea of the arena though I have tried to battle the first with lower level pets. Any tips?

When it comes to arena battles, you really need to pay attention to the battle log, and the symbols under the boss, because each boss have a special move, that if you don't avoid, will make it practically impossible to defeat.

Some moves require you to block, some you have to be quick, and some you'll need to use the right strategy to choose the right opponent to defeat first.

I'll recommend you reading this guide https://www.sylestia.com/forums/?thread=26551. It's the most detailed guide on the arena. I heard one of the strategy for the later battle is outdated though... haven't played in the arena for a while now, so I'm not sure. But this will definitely help you through the level 25 bracket for sure.