When I have both Adept Watershaper on the board and the mana dork Great Forest Druid, if I move to block with Druid, and have no spells to cast, Arena skips past allowing me to tap Druid for mana, thus making it indestructible. Instead it just moves to damage and assuming attacker has power 4 or more, kills my Druid. I should be able to tap for mana, waste the mana, and make Druid indestructible.
I haven't tried taking control of all steps and do it manually.