This is a tough one to call as Longshot is such a wildcard. The ability to alter probability is a might one indeed and used to it's full potential has almost limitless power. Reducing the chance that any strikes (melee or ranged) will hit, increasing the chance that his team will hit. Or hell even suddenly increasing the chance that Cyclops or Havok will suddenly suffer a fatal heart attack and die without a punch being thrown. I'm not 100% sure of the limits of Longshot's powers but if he can reallly mess with it, there's not much he couldn't achieve.