There are a few important things to consider when it comes to setting up the perfect team:

Fighters' Fate: this is unlocked for having various fighters in the team. These are all extremely varied, but offer nice bonuses. So definitely keep an eye on them and have them in mind when deciding on your team

Team composition: you can't go with a team full of Tanks, nor with a team without a Healer (in most cases). Find out what's the best composition (I recommend trying two Tanks, one Healer and three Damage Dealers - or you can go without a Healer for most regular fights)

Fighter's abilities and skills: Each fighter has different skills and abilities. The ultimate move is extremely important here, so make sure that you select those with the most devastating Ultimate moves.

Fighters' rarity: The rarer the fighter, the better their stats and overall performance - this is the general rule of thumb. So always try to prioritize your rarer fighters and work on keeping them at max levels.