A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from the Wayback Machine (its copy), so posts it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Does Isawa Tanaka have to be at the battlefield to use his ability or can he do so from your home?
Isawa Tanaka
Phoenix Clan Air Shugenja
Elemental Battle: Any number of times per turn, target and bow one of Tanaka's Spells: Target a card at the current battlefield. Reduce its Force to 0.
Does Isawa Tanaka have to be at the battlefield to use his ability(?)
In general, cards do not need to be in the battle to use abilities. Exceptions: If the card says "opposing," "in the battle" or something like that, it has to be there.
But otherwise, if you have a unit in the battle (Presence), and it affects the battle (Relevance), actions are legal.