Creatures you control can tap to produce at least ({3} magic symbol) ({R} magic symbol) .
Steps
Activate all mana-producing creatures you control by tapping them, adding at least ({3} magic symbol) ({R} magic symbol) .
Cast Walking Ballista by paying ({0} magic symbol) .
Walking Ballista enters the battlefield and dies as a state-based action due to having zero toughness.
Intruder Alarm and Mortuary trigger.
Resolve the Intruder Alarm trigger, untapping all creatures.
Resolve the Mortuary trigger, putting Walking Ballista on top of your library from your graveyard.
Activate Hidetsugu's second ability by paying ({2} magic symbol) ({R} magic symbol) and tapping it, exiling Walking Ballista from the top of your library and allowing you cast it this turn.
Repeat an arbitrarily large number of times.
Cast Walking Ballista from exile by paying an arbitrarily large amount of mana, causing Walking Ballista to enter with half that many +1/+1 counters on it.
Activate Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to any target.