1) Kill that minion ASAP! Do whatever it takes to get rid of him fast. Then heal yourself if you need it.
2) Beware of the Gnomes! spell that he casts, as it can stop you from being able to heal yourself. One thing that can help you survive that is having "Cleanse Charm" treasure cards in your side deck.
3) Be sure you have the best possible gear. Waterworks, House of Scales, Atlantis, or Wintertusk crafted are your four best options at level 70. The first three I mentioned are drops from dungeons, but the Wintertusk gear is crafted and well worth the effort if you don't want to (or can't) do hard dungeons.
4) Be careful when choosing to use your minions. Sometimes the best possible guy is your little puppet, as he is determined to hit every turn and doesn't use any pips to summon. Your strongest options are Vassanji or Talos, but summoning them can cost you a lot of pips plus several turns. Honestly, I don't ever use these.
5) Blade up and use Medusa or your Basilisk spell Mika Skarka. Use two or three blades at once, also traps for Medusa. Use your bubble. The key is to get as much power behind your hits as possible.
6) After you hit, pause to consider your health levels. Heal yourself if needed, especially if you're down below half on health points.
7) Blade up and hit him again if needed. Check your health. Repeat as needed until you're finished.
8) If all else fails, buying a henchman can help. I recommend the Life henchmen, between levels 50-70 (because they are fairly cheap yet still effective). The level 70 gorillas are best at focusing mostly on healing and not so much on hitting. I think they're 75 or 100 crowns. You can earn a small amount of crowns by watching videos or doing the Daily Assignments.