Completing daily activities is a great way to never go out of game's most important resources, and the same applies for achievements, always keep them in mind.

The blue meter underneath the health bar indicates who is the closest to being next, so targeting the opponent's character who is close to being fully charged is the best viable option.

Completing every main achievement of the game will enable you to unlock Dart Vader, as you'll be awarded 80 Shards and that's how much you need to add the best villain of all times to your roster.