Hey! Not the original poster here, but thought id give my opinion :p For the damage over time, you could add evil spirits/poison that you cant defeat and explain that they deal constant damage (that would explain the hp degen). Perhaps a tiny 2-3 step tutorial at the start, with a small scroll with 1-2 lines of text (eg: 1. click on monsters to attack. Careful, they attack you back! (with 1 monster on the grid that they kill in one shot) 2. Poison deals constant damage. (Grid with no enemies, perhaps a heart animation draining?) 3. Collect money for upgrades! (and a few coins on the ground you pick up with mouse)). That would be 30s-1min and explains the mechanics (as far as i´ve seen, ive only played like twice). Also, on the health regen/enemy kill for hp upgrades, you could add some extra text (since people actually do read those) that says maybe Poison still affects you. The longer you play, the more damage it deals. or Enemies will still damage you when you hit them.
Hope this helped!