Poison as used by enemies is strong, but it's because they can often create multiple Poison gems, and there are multiple effective counters to it. The most easily accessible counter available to all character builds is not accessible until after you've entered the forest; however, you do not need to have beaten the boss to acquire it. If you find you're having trouble, craft and purchase the Antidote Gel then come back. Which is not to say the boss isn't hard -- it is actually quite a difficult boss.
You also mentioned that the MC doesn't have effective Poison or Bleed spells. For Poison, this is true, but MC gets mostly the same Bleed skills that enemies do, and individual Bleed gems are typically stronger than individual Poison gems. Also, the MC can get Burn, which can be stronger than either (depending on build) and the MC has the ability to place several Burn gems at once. If you're looking to fight DoT with more DoT, try a Fire build. The Treant is also weak to Fire, including Burn since it does Fire damage (Poison and Bleed do Physical damage).