Mime depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Mysterious Treasures)
Cast it early and pair this angle cleric with effects like Resplendent Angel to really get the ball rolling
The Treasure token is a nice curve helper in the early game and the lifegain has many advantages
While giving your opponent a whole new hand each turn sounds bad in theory, it actually breaks up their ability to sculpt and plan for future turns
Even though the humble Negate may seem like a better exchange of mana-to-possible-counter, remember that opponents are often tapping out to cast their huge Fireball and not expecting to see a Spell Pierce countering their spell as well as a Counterspell would, anyway
Adagia, Windswept Bastion The ability to copy any enchantment or artifact that you control is really powerful