I'm still on the noob side but I find some tips very useful:

In battle

1. ALWAYS bring a Tank
Originally I stupidly brought two offensive pets with me. They died a terrible death. It costs a bit but having a tank (Earth, Water, Shadow) in the middle slot is worth it.
2. Bring a Cannon too
Wind and Fire are the cannons. They deal higher damage and don't do anything else. They help finish fights much faster.
3. Target one enemy
When there are multiple enemies, target one. If you have each of your pets target a different enemy, they'll die more slowly. If you knock them out one by one you'll receive less damage.
4. Use mend, healers, and elixirs
You need to alternate between the three. Mend is the most effective, but has a huge cooldown. Light heals less, but can be good when it's not an emergency. Elixirs are expensive, but can save your life if used at the right moment.
5. Use block
Block seems useless, but some bosses have dangerous attacks that need to charge. If you come across an enemy that has an effect saying they are charging for something, have your middle pet block. This is especially useful for avoiding negative effects.

Suggested Teams

This is my team:
Shadow Tank: Can heal himself and deal decent damage, making survival relatively easier
Air Cannon: The strongest damage element in the game, decent health
Light Healer: Can be good in longer fights, decent health

Another good one:
Water Tank: Can provide good buffs to his team and himself
Fire Cannon: Multi-hit and decent damage, decent stats
Light Healer: Light is the only healer element

About Equipment

1. Use the ones with high req level
They tend to be stronger and have better stats. Still remember, the Legendary and Epic equipment are the best.
2. Use biased armor
While the ones that give equal stats seem good, NO. If you have a melee pet, then boosting intelligence is useless. For melee, look for Physical Mitigation, Physical Damage. Strength. For magic, look for Magical Mitigation, Magical Damage, Intellegence.