Use your Bard's abilities to buff your allies to give them advantage, or to break your enemies to give them disadvantage. Bards can also put enemies to sleep, make them fall prone with laughter, and sling horrible insults like "you have a visage well suited for scroll-writing." This will make your allies less likely to take damage, and more likely to deal more damage themselves. A bard can also heal, so at least in the early game you can run 3 dedicated DPS and a bard and have no issues in most situations.