Assassin has “three attacks (of one strike each) all against the same character,” while Ruse’s second ability states that it is “playable on a scout facing an attack. No strikes of the attack may be assigned to the scout.”
If Assassin is played on a company that only contains a single scout, and the scout uses this second ability of Ruse before strikes are assigned for the first attack, the hazard player would be unable to assign the one strike of the first attack and that attack would be ineffectual. However, since each strike of the Assassin’s three attacks must be assigned to the same character, the second and third strikes cannot then be assigned to the scout since the first strike wasn’t assigned to the scout, and the Assassin would be discarded without having assigned a single strike.
For reference, ICE’s NetRep answered a similar question in ICE 117:
Suppose a single character company is facing an Assassin. MSTY is played preventing the strike of the first attack from being assigned. Assassin says, “Three attacks all against the same character”, and since the first attack was against no character, does this mean the character cannot be targeted by the second or third attacks either?
Yes.