Maybe even snag some whale friends that can one turn that fool unless you are going for legit kills
It's the one boss fight in the story where you can't bring a friend, unfortunately (since it takes place in a town).
Rob's suggestions are good - if you have a breaker, that would be really useful too. This weekend if you're using cactaurs to level, favor individual units over spreading them out - leveling higher requires more XP but offers new abilities and bigger stat improvements (Chiz or Exdeath would be the best damage dealers for you).
Rain might be a good bet too as he's quick to level and has a good breaking ability at 6* with Undermine, but he'll probably take a long while to get there.