The Equip Spell Cards used for "Guardian Ceal"'s effect must be activated and equipped to "Guardian Ceal" by "Guardian Ceal"'s controller in order to be on that player's side of the field.
Sending 1 Equip Spell Card equipped to "Guardian Ceal" to the Graveyard is a cost.
When you send "Big Bang Shot" to the Graveyard for "Guardian Ceal"'s effect, your opponent can chain to this effect, and then "Guardian Ceal" is removed from play.