No-one said they should be able to take out (non-Minion) enemies in one hit. They said that, by operating as a team and focusing fire, they should be able to take out enemies in one round. Usually, you want to gang up on one enemy at a time until it's dead, preferably the one that does the most damage or has the greatest ability to cause debilitating conditions. Then move on to the next one and do the same.