There are multiple ways of boosting your damage higher. Perhaps the easiest is a Wrathful or Reaper Hellhorned Prince. For a Wrathful Prince, set them up in front so they take plenty of hits, as each hit adds a stack of Rage. (+2 Attack per stack, stacks decay every turn) Make sure they are still getting kills to get the armor on slay. For a Reaper Prince, stick them in the back line and every kill they get increases their attack on that stage. For an even easier time, combine this with a Brawler upgrade to get Multistrike.
Outside of this, there are plenty of ways to boost attack, such as Rage from other sources, Awoken spells such as Steel Enhancer or Sharpen, or unit upgrades. There are even a few units who can deal over 60 damage with no assistance, though it will take a while to unlock those.