Instant, 1R (2) Guerrilla Tactics deals 2 damage to target creature or player. When a spell or ability an opponent controls causes you to discard Guerrilla Tactics, Guerrilla Tactics deals 4 damage to target creature or player.1
Illus. Ron Spencer
If it is discarded, the damage done by it is done by this card due to an ability and not by a spell. So Suffocation cannot be used on that damage. [D'Angelo 1996/11/11]
The second ability will trigger even on an optional discard caused by an opponent, as with Chain of Plasma. [D'Angelo 2003/09/08]