Excepting for times where you want to focus on an overpowered enemy that can do a lot of damage, consider attacking the enemy that you can destroy the fastest first. Do this before it attacks or at least before it manages can attack twice. Target one enemy at a time and destroy them before moving to the next. You can include area of effect attacks while doing so. During Boss battles, destroy the supporting troops they bring with them then focus on the Boss at the end.
There are many classes available. Certain classes have advantages over the others. Try to bring heroes that have an advantage over your opponents.
Make friends to get access to heroes that will help in battle as well as more silver from the heroes you hire from your them and when your own heroes are used in their battles.
Spend gold to get Superior Power Cells. Use them to get new heroes outside of the free hero obtained after seven days. Note that there is a chance that a cell will not provide a hero. If you are satisfied with your heroes, spend the gold instead on Scouting mission refreshes.
After seven days of game play, you will get the chance of adding a free hero to your team. Choose one that will boost your lineup; for example from an element you do not have yet or one that is better than your worst hero.
Carefully the ISO compounds obtained during battles to get extra experience for your heroes. Always use them based on elements to get the largest experience point gain. Do not invest all your XP boosts on early characters because they may be replaced by better ones later.
Watch the XP meters and collect your task rewards when close to leveling up. After leveling up, your energy replenishes. Do not level up when your energy is full or it will just be wasted.
As you progress you will unlock additional skills for your characters. However they will not be able to bring them all into battle. Choose the best ones after taking their upgraded effects and how they help your team into account. A skill might be good on its own, but another might be better overall if it results in a big boost to your teammates or overall large bonuses.