The user gennie has a helpful battle guide. I copied my response from the FAQ section here:
https://sites.google.com/site/marabattleguide/home
Q: How can I make my pet stronger?
A: To make it stronger, you'll need to train it in the Gym.
Gym stats include: Level, Strength, Defence, Speed, and Health.
*Level: This stat determines the cost and time it takes to train the other gym stats. You should keep this as low as possible, and only train it when necessary.
*Strength: You can train this stats to double your pets Level, plus 1. This stat combines with Speed, to aid in boosting your weapons damage.
*Defence: You can train this stats to double your pets Level, plus 1. This stat is used currently to earn Kirin prizes, and for a few select battle related Hidden Avatars. It may become more useful later on. Train it.
*Speed: You can train this stats to double your pets Level, plus 1. This stat combines with Strength, to aid in boosting your weapons damage.
*Health: You can train this stats to double your pets Level, plus 1. This stat is mostly self explanatory. Your pet needs it to take hits from its opponents. The more health your pet has, the more damage it can take each move in battle.
It goes on to say \"Alternatively, you can also use boosters to increase your pets stats. Attaching stronger weapons and wands can also help with the amount of damage your pet can deal and heal.\"
But I wouldn't recommend using boosters until your pet gets stronger, or rather, until it takes a longer amount of time to train at the gym.
Here's a link to the gym:
https://www.marapets.com/gym.php