"Victim" refers to the target affected by Walking Bomb set to explode.
This upgrade does not remove the Disorient effect in the way that a "detonator" ability like Spirit Strike would, rather it simply uses it to check for whether to issue double damage or not. This leaves the Disorient effect able to be exploited for a cross-class combo.
The 200% effect only applies if the victim is Disoriented before Walking Bomb is cast. Once the effect is activated, however, it will maintain double damage until the spell expires, even if the cross-class combo setup is changed (i.e. to Stagger or Brittle).
Despite the ability's description, the damage inflicted is only a rough approximation of the sum total dealt over the duration of Walking Bomb, with a fractional amount dealt every couple of seconds until the ability expires a la Despair.
This upgrade can be an effective way of finishing off a weakened victim (e.g. Enemy: Health < 10%) for the purposes of setting off the primary Walking Bomb explosion, while withdrawing all party members out of the blast radius. This would only be an issue on Nightmare difficulty, however, as the other difficulties do not have friendly fire.