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The primary way to acquire credits is to participate in battles. Each point of damage you inflict on your enemies will be earning you credits; you don't have to kill an opponent or to win a match. Therefore, it's better to face a superior enemy and damage him as much as you can before you die, instead of running around the map and trying to get a Draw Match.
Should you decide that you want to upgrade your robot more quickly – you can also purchase Credits from the Official Website of Guns and Robots.